The Repository Abyss Report: Into the Dark

The Dead Vents Deep-Sea Mining Wants Weren't Dead

October 11, 2026 · Runtime 24:58 · Watch on YouTube

On 21 July 2022 a NOAA robot reached Moytirra, the only hot vent field known between the Azores and Iceland, and was sent to study the part that was dead: the inactive sulfide chimneys that deep-sea mining targets. Two chemical sensors, one on the robot and one 20 m above it, show where dead ends and alive begins, and why a camera cannot.

Chapters

  1. 0:00 A vent at 54 degrees
  2. 1:02 Moytirra, found in 2011
  3. 2:23 How a vent builds ore
  4. 3:05 Sent to the dead chimneys
  5. 4:07 Why mining wants them
  6. 5:31 The sensor that reads the water
  7. 6:18 Skeleton shrimp
  8. 7:14 A message from shore
  9. 7:52 Fool's gold
  10. 8:49 12:35, the first drop
  11. 9:44 Green stalactites
  12. 11:02 Sample 03G
  13. 11:55 The nozzle breaks
  14. 12:11 Fuzzy rocks nobody can name
  15. 12:59 A possibly undescribed coral
  16. 13:24 A second sensor, 20 m up
  17. 15:05 White chimneys
  18. 16:31 Not inactive completely
  19. 17:32 The temperature probe
  20. 19:56 The Fomorian
  21. 21:19 -66 mV, inside the plume
  22. 22:19 Inactive is a measurement
  23. 23:52 What is still open
  24. 24:27 Next: the holes in a line

Transcript

A vent at 54 degrees 0:00

Two thousand nine hundred and forty-seven metres down, on the Mid-Atlantic Ridge, a small chimney is breathing out water at fifty-four degrees. It belongs to the only hot vent field anyone has found between the Azores and Iceland. And on the day a robot finally came back to it, the robot had been sent to look somewhere else. It had been sent to look at the chimneys that were dead.

So here is the question this episode answers. How do you know that a chimney on the sea floor is dead? It sounds like a question for a geologist. It is now also a question for the mining industry, because the dead chimneys are the ones deep-sea mining wants. The answer comes from one dive, minute by minute: what the camera saw, what the scientists said, and what a small sensor on the robot read in the water. Every moving picture in this episode is NOAA footage from that dive. Every number comes from the dive record, a published paper, or our own reading of the robot's raw data.

Moytirra, found in 2011 1:02

The place is called Moytirra. Irish scientists found it in August twenty eleven, from the research ship Celtic Explorer, with a robot called Holland One, about three thousand metres down, north of the Azores. They named it after a battlefield in Irish mythology, the Plain of the Pillars, and its biggest chimney after a giant from the same story. Their paper, in twenty thirteen, counted four venting sites and a chimney eighteen metres tall. It also called the field unusual. It sits on basalt, perched halfway up the wall of the ridge's central valley, presumably heated by magma off to one side.

The Mid-Atlantic Ridge pulls apart slowly here, a few centimetres a year. Hot vents on slow ridges are rare and far apart, and this one is alone. The nearest known vents to the south, off the Azores, are about nine hundred kilometres away. The NOAA ship Okeanos Explorer reached Moytirra on the twenty-first of July, twenty twenty-two. The robot, Deep Discoverer, went in the water at ten twenty-one in the morning, universal time, and touched bottom at twelve fifteen, three thousand and thirty-eight metres down. One of the geologists on the live call put the difficulty in one comparison.

How a vent builds ore 2:23

First, how a vent works. Sea water sinks into cracks in the crust. Deep down, near hot rock, it heats far past boiling, but the pressure keeps it liquid. It dissolves metals out of the rock, iron and copper among them. Then it rises, and when it meets sea water near freezing, the metals drop out as sulfide minerals. Those minerals build the chimney around the jet. When the heat moves on, the chimney goes quiet. It rusts, it cracks, it falls down the slope. Over thousands of years the rubble piles up into a mound of metal ore. The geologist on the call explained the hot end of that story live.

Sent to the dead chimneys 3:05

The dive plan for that day has one line under purpose. To explore and characterize the organisms living in the hydrothermally inactive sulfide habitats. Not the hot chimneys. The dead ones. The dive summary even says why, in brackets: potential mining targets. Two minutes after landing, the science team said out loud what they were hunting for.

Why mining wants them 4:07

Here is the logic of deep-sea mining. An active vent field is small. The largest known ones cover less than three hundredths of a square kilometre, and they are crowded with animals that live nowhere else. A mound that has gone quiet may have grown for thousands of years, and it can be hundreds of metres across. It is cold, it is rich in metal, and it is assumed to be nearly empty. In twenty nineteen the biologist Cindy Lee Van Dover reviewed everything known about life on these inactive mounds. Her finding was that only a few studies had ever measured it.

The first thing the camera found was colour. The robot came down on a slope of orange and brown mud, iron and sulfur that had settled out of the chimneys upslope over a very long time. Scattered across it were small white shells. For a moment, nobody on the call could place them.

The sensor that reads the water 5:31

While the camera looks, something else is reading the water. Under the back of the robot there is a small sensor that measures oxidation-reduction potential, O R P, in millivolts. Ordinary deep sea water is full of oxygen, and the reading sits high. Vent fluid carries reduced chemicals, dissolved iron, hydrogen sulfide, hydrogen, that do not belong in oxygenated water. When the robot passes through even a trace of them, the reading drops. Those chemicals react away within about a kilometre of where they come out. So a drop means a source, and a close one. The robot logged this sensor twenty-four times a second for the whole dive. We decoded the raw file.

Skeleton shrimp 6:18

Fifteen minutes in, the first animals the team could name. On a dark rock, thin shapes stood upright and swayed, like stalks in a wind. Skeleton shrimp. They are not shrimp at all. They are amphipods, holding on to the rock with hooked back legs and grabbing at the water with the front ones.

A message from shore 7:14

A few minutes later, the same rock had tubes on it, worm tubes, and a message came in from shore. It came from Cindy Van Dover herself, the biologist who wrote that review, watching the live feed.

Fool's gold 7:52

Half an hour after landing, a little further up the slope, the lights caught a boulder that did not look like the rest. It glittered. For a moment, nobody wanted to be the first to say what it looked like.

12:35, the first drop 8:49

Now look at what the sensor was doing in the same minutes. On the way down, the reading crept up slowly, from about one hundred and forty-two millivolts to one hundred and sixty. That is the electrode settling in. Then, at twelve thirty-five, on this slope that everyone on the call was describing as inactive, it fell. About fifty millivolts in half an hour, down to one hundred and seventeen. There is a catch, and it matters. In the same minutes the turbidity sensor jumped too. The robot was nosing into sulfide mud and stirring it up. On its own, the sensor cannot tell a trickle of vent fluid from the robot's own cloud. So this is the first thing this episode will say carefully. The sensor says something is in the water, right here at the rocks. It does not say what. Later in the dive, a second witness will help.

Green stalactites 9:44

Then the colours got stranger. Green crusts on red rock. And under one ledge, small green stalactites, hanging down as if something had dripped and set. Someone on the call said it was the most colourful geology dive they had ever been on. A chemist ashore had a name for the green.

Sample 03G 11:02

At thirteen fifteen, three thousand and thirty-one metres down, the arm lifted a rock streaked with that green, and the skeleton shrimp were still holding on. Eight of them went into the sample drawer with it, catalogued as sample zero three G. The water around it was three point four nine degrees. That is the dead field. Cold, coated in copper, and not empty.

For the next three hours the robot climbed. The slope was talus, broken blocks of old chimney that had fallen down the hill, mixed with basalt. The team stopped often, because the request from shore was unusual. Look closely at the surfaces of the rocks. Look for small animals, the kind a normal survey drives straight past. On almost every rock they looked at, there were more skeleton shrimp.

The nozzle breaks 11:55

Not everything went to plan. At fourteen fifty-three, the suction sampler, the vacuum hose the robot uses to collect soft animals, broke at the nozzle. On a dive about small animals, that is the tool you least want to lose.

Fuzzy rocks nobody can name 12:11

All afternoon, one thing kept coming back. Rocks covered in fine white strands, waving in the current. The team called them fuzzy rocks. Some of the fuzz was tree-shaped foraminifera, single-celled organisms that build branching tubes. Some of it was tiny tubes of mud, a centimetre or less, and nobody could say what had built them. They tried for hours to find a fuzzy rock small enough to lift. Just before four, they found one.

A possibly undescribed coral 12:59

Halfway up the slope, the only large animal of the day that was not a vent specialist. A bamboo coral, a single unbranched stem, rising off a rock outcrop, with brittle stars wrapped around it like ribbons. The arm took a piece. In the expedition's list of samples, the reason for collecting it is three words long. Potential undescribed species.

A second sensor, 20 m up 13:24

Back to the sensor, and now the second witness. The camera sled that hangs about twenty metres above the robot carries the same kind of sensor. Over the whole inactive slope, for almost four hours, its reading barely moved. A slow drift of a few millivolts, nothing more. Down on the rocks, the robot's sensor dipped again at five past three, to one hundred and twenty, and again the turbidity jumped with it. Whatever the robot was reading down there never reached twenty metres up. Then, at four o'clock, both lines began to fall, together. When they did, the robot had not yet seen a single active chimney.

Just before four o'clock, after almost four hours of climbing through rubble, tall shapes came out of the dark, standing on the very edge of the slope.

White chimneys 15:05

The old chimneys were red and brown, rusted iron. Ten minutes later the colour changed to white. White here means anhydrite, a calcium sulfate that only forms where vent fluid is hot. In cold sea water it slowly dissolves away. The team had a map from a Spanish survey of twenty nineteen, which separates the old, oxidized chimneys from the white anhydrite ones.

Not inactive completely 16:31

Around the white chimneys there were eelpouts, long pale fish from a family with members that live only at vents. On shore, Cindy Van Dover asked for a temperature reading. She bet there was a thermal anomaly. Then the pilot swung the lights onto the tallest chimney in view, one the team had just been treating as dead.

The temperature probe 17:32

The robot carries a temperature probe for exactly this. At sixteen twenty-eight the arm brought it into the shimmer at the base of a white chimney. The highest temperature NOAA recorded at this vent was fifty-four degrees. Water at the bottom here is three and a half. Black smokers can pass three hundred and fifty. This one is weak. It is also, unmistakably, alive.

On the active chimneys, the animals changed completely. Peltospira snails, a genus that lives only at vents. Mirocaris shrimp, which depend on bacteria that feed on the vent's chemicals. Segonzacia crabs. The two thousand thirteen paper found that this community looks south, to the Azores vents, not north toward Iceland.

The Fomorian 19:56

And then, turning left, the biggest structure of the day. At least ten metres tall, shaped like organ pipes, capped with rounded beehive forms where warm water seeps out through the walls. The team believed it was a chimney the Irish expedition had named back in twenty eleven, after a race of beings from that same myth. The Fomorian.

-66 mV, inside the plume 21:19

Now both sensors, the whole dive, in one picture. The sled's line, twenty metres up, is flat for four hours over the slope everyone called inactive. The robot's line, at the rocks, flickers. At four o'clock both collapse, and over the active chimneys the sled falls furthest, to minus one hundred and fourteen. Then, at five twenty-six, the robot left the bottom and began to rise. Seven minutes later, eighty-one metres above the sea floor, its own reading hit the lowest point of its dive. Minus sixty-six millivolts. In the same minutes the temperature rose by a fifth of a degree, and the turbidity sensor jumped. Chemistry, heat and particles at once. Edward Baker of NOAA counts chemistry and particles together as the mark of a high-temperature vent field. It is the plume, the warm, mineral-laden water rising off Moytirra. The camera saw open water.

Inactive is a measurement 22:19

So, was the slope dead? From twenty metres up, yes. The sled's sensor never moved over it. Down at the rocks, the robot's sensor dipped twice, and both times it was stirring up mud. So the honest answer from this record is that nobody can tell yet whether that rubble still leaks. In twenty seventeen, Baker wrote that drops in this reading can find places that no longer build ore but still leak warm fluid. Van Dover put the same problem from the other side. A deposit described as inactive can later turn out to have active vents in it. At Moytirra, the line between the two moved inside one afternoon. The word inactive is not something a camera can see. It is a measurement, it depends on where you put the sensor, and on this slope nobody has made it yet. And one of the scientists who found Moytirra in twenty eleven was watching the feed that day.

What is still open 23:52

Here is what is still open. First, the skeleton shrimp on the dead rock. Are they a special community of old vents, or ordinary deep sea animals moving in? The samples are in alcohol and formalin, and we found no published answer yet. Second, the mud tubes on the fuzzy rocks, which nobody could name. Third, the bamboo coral listed as possibly new. And fourth, the measurement this dive points at. A map of this sensor across the whole quiet field, before anyone decides it is empty.

Next: the holes in a line 24:27

This expedition had one more mystery. Two days later, on a volcanic ridge about five hundred metres shallower, the camera found lines of holes in the mud. Neat rows, evenly spaced, each with a small pile of sediment beside it. The same pattern had been photographed on this ridge in two thousand four. Nearly twenty years later, nobody has seen what digs them. That is the next Abyss Report. The holes in a line.

Description and sources

EX2205 dive 02 spent 5 h 11 min climbing the Moytirra vent field, from 3,038 m to 2,938 m. Its written purpose was the inactive sulfide habitats, which the dive summary calls potential mining targets. On rust-red rubble and copper-green rock the team found skeleton shrimp, tubes of mud nobody could name, and a bamboo coral listed as a potential undescribed species. We decoded the raw CTD files of the robot and of the camera sled 20 m above it; both carry a PMEL oxidation-reduction potential sensor. Over the inactive slope the sled stayed flat for four hours, while the robot dipped twice at the rocks as turbidity rose: faint seepage or stirred mud, this record cannot tell. From 16:00 both collapsed over the active chimneys. At 16:17 the team said a chimney they had treated as dead was not inactive completely. Rising at 17:33, 81 m up, the robot crossed its lowest reading, -66 mV, with temperature and turbidity rising too: the plume. Highest temperature NOAA recorded at the vent: 54 °C.

PRINT-READY, FROM THIS CHANNEL

Read This Dive, Vol. 01 — the method, and five dives read with it

https://therepository.gumroad.com/l/read-this-dive

PRIMARY SOURCES

Abyss Report reads the deep ocean through the record that resolved it -- instrument, date, position and the primary sources above.

About the material. Every moving frame is NOAA Ocean Exploration video, public domain, credited on screen with dive, depth and position: EX2205 dive 02, Moytirra vent field, Mid-Atlantic Ridge, 21 July 2022. Depths come from the vehicle's 1 Hz track; the catalogue gives the longitude without its sign, corrected against the dive track. Several cuts drop the narration and play the original control room audio; each window was re-transcribed in isolation twice and used only where both passes matched. The ORP, temperature and turbidity readings are our own decode of the robot's and the camera sled's raw SBE 911plus files, checked against the dive summary. Every diagram is drawn in code from the sources listed.

#DeepSea #NOAA #OceanExploration #DeepSeaMining #HydrothermalVent