A few questions about the DEEP parts of the ocean

Factual Questions
1

I have long had an interest in the ocean, particularly the parts where it gets ridiculously deep (I think this stems from the fact that, for a long time, I couldn’t swim and water two metres deep was enough to frighten me; as a consequence, I used to like freaking myself out reflecting on the fact that there were parts of the ocean where the water was 10000 metres deep or something even more ludicrous than that :wink: ). Anyhoo, I have a number of questions relating to the abyssal depths. Answers to any or all of them would be greatly appreciated.

Firstly, how deep could a human being swim/dive before the mass of the water above them would crush them ie what is the “crush depth” of a human being (I hope I have used that wonderfully evocative phrase correctly here)?

Given how cold the ocean can get in those regions that no sunlight reaches (ie just above freezing), why doesn’t the water in the really deep parts of the sea ever freeze, particularly in the polar regions?

Purely out of morbid curiosity, what would happen to a person who was stupid/suicidal enough to leave a submersible, bathyscaphe etc at a depth at which the water pressure was crushingly high? I know they would probably be squished, but what exactly would that entail? Would it likely be messy and distressing to view?

Conversely, what would happen to a deep-sea-dwelling lifeform that was brought to the surface? I have heard that this sort of dramatic depressurization usually proves fatal to the unfortunate organism which experiences it, but what actually happens? Would the organism balloon up and explode, or would its demise be somewhat less dramatic than that?

Have there been any more descents (manned or otherwise) to the bottom of the Marianas Trench since Picard and that other guy (sorry, his name eludes me right now) made their historic one in the bathyscaphe Trieste in 1960? Has man descended to the bottom of any other oceanic trenchs? Just how many trenches are there anyway? On the subject of the MT, I remember some guy telling me that, because the water there was so deep, it was actually black. A nice story, but is it true?

Do underwater explorers still use bathyscaphes (or have they been superseded by something better)? Are there any revolutionary technological breakthroughs on the horizon in the area of deep-sea exploration?

2

Maximum density of water is reached at +4 C. As water becomes colder, it becomes less dense. Therefore colder water, and particularly ice, will rise. These properties ensure that water will always freeze from the top, rather than the bottom.

3

WARNING: definitely WAG territory here…
First, it would be pretty hard to actually leave the submersible at such a depth. In order to leave, the person would first have to push him/herself out against the extreme pressure and water rushing into the submersible. Using an airlock-type exit would kill them before they exited, and perhaps their best chance for spectacular suicide would come from being ejected out of a torpedo tube.
As far as what exactly it would entail; no one really knows. Most likely the rapid expulsion of air and fluids as the lungs collapse would not make things pretty. At extreme pressure, the crushing of the body shape into something relatively unrecognizeable may be possible, but AFAIK, no one has ever witnessed a death of this sort. That’s just a guess.

The demise of such deep-sea creatures is usually somewhat less dramatic than ballooning up and exploding (or at least so I hear second-hand). They’re often quite fragile, as a result of having very little of anything resembling a solid wall to run into down there. Often just colliding with the wall of a collection jar or a net is enough to puncture their membranes and kill them. Benthic (bottom-dwelling) creatures are sturdier and often have shells. Their demises are also less dramatic than the balloon example, but I hear they tend to arrive on the surface looking rather “deflated.”

Jacques Piccard and Donald Walsh made the descent in 1960. Their most famous dive in 1960 was the deepest of a series (possibly 3) of dives to the Marianas Trench. Some information about the Trieste. The bathyscaphe was retired soon afterwards in 1963. In fact, it seems that shortly after the deepest dive the modifications that made it possible for Trieste to go that deep were removed. AFAIK, no one has returned to the “Challenger Deep” of the Marianas Trench since. Humans have been to the bottom of many of the other ocean trenches. Usually trenches are considered areas of the ocean deeper then about 4,000-5,000 m, so there are probably about a dozen or so, including the Mariana, Japan, Aleutian, Kuril, Puerto Rico, Peru-Chile, Java, and Phillipine Trenches (I’m sure there are some more… a google search may turn them up). About the water being black… everything is black down there. There’s no light. All the water above has absorbed or reflected more than 99.9% of the sunlight before it reaches that depth. Of course, if you bring your own illumination, the water’s clear for a short distance, just like water anywhere else. There’s nothing special about the water there; it’s just that there’s so much of it between you and the sun.

Yes, but the definition of bathyscaphe is somewhat general. “Submarine” or “submersible” works equally well in casual conversation. Some of the current workseahorses of the deep include Alvin, Sea Cliff, and Shinkai 6500. (None of the aforementioned could survive at the bottom of the Marianas Trench). Currently, much attention is being paid to the development of new Remotely Operated Vehicles (ROVs a.k.a. AUVs) which are easier to use and less expensive than manned submersibles. There are also developments in manned exploration such as Deep Rover, Deep Flight, and others.

4

More WAG, but from what I know the change in density in water between the surface and the bottom of the ocean is miniscule. As you are alost all water, there would be little visible change. The only real concern would be the air in your lungs, so if you had a full breath of air you would probably be unable to stop the sudden rush of water into your lungs. If anything is going to implode (it would not explode), there has to be somewhere to go, but there isn’t. As for the animals coming up from the bottom, they do not even have air in their lungs, so they have nothing to explode.

I am not even sure the pressure itself would kill you. You would, of course, have your ear drums blown in, but what else would it do?

5

I’ve read that there is nothing dangerous about the pressure itself, up to a certain limit. Whether it’s true or not, I’m not sure – I don’t know if the scenarios for deep-water habitats that have been dreamed up by fiction authors have ever been tested in reality. It does seem to be true in fiction, though, so I’ll relate what the fiction I’ve read says. :slight_smile:

In OceanSpace and Sphere and The Abyss and other books/movies dealing with deep-ocean habitats, the habitats are extremely flimsy and would be instantly crushed by the water around them if they were filled with air at sea-level pressure.

But, fortunately for all concerned, the habitats aren’t at normal pressure, they’re filled with a high-pressure atmosphere where the oxygen is greatly reduced and the nitrogen (I think it’s the nitrogen, anyway) is replaced with helium. This prevents the oxygen from becoming extremely corrosive, and prevents the nitrogen from becoming intoxicating. So the pressure inside the habitats is equal to the outside pressure, and people are still able to function normally (except for the unusual atmosphere, of course). In fact, they can go outside the habitat and swim in the surrounding ocean without a problem (except for the extreme cold).

But, according to The Abyss, at extreme depths (measured in multiple kilometers, IIRC), the pressure is so great that it starts deforming neurons in the brain and other bodily structures or tissues, and you start to experience Bad Things like insanity and death. I believe what Orson Scott Card and/or James Cameron actually wrote was that the pressure starts “squeezing your synapses together” or something like that.

Again, the sources are all fictional, so make of it what you will. Sure sounds interesting though.

6

PBS showed a series called “Scientific American Frontiers” hosted by Alan Alda. In one episode, he interviewed Robert Ballard (who found the Titanic wreck, among many other accomplishments). Ballard talked some about the how the depth would kill you if a submersible failed.

In one part, he talked about the discovery of black smokers in the Pacific. They stuck a probe in to measure the temperature and the probe melted. This was very exciting until the pilot pointed out that the windows were made of the same material as the probe. I don’t recall exactly what Ballard said, but it was basically that if the window failed and the hull lost integrity, the submersible would implode and the whole thing would be over in under a second. In this case, the people in the sub would be killed by all that moving metal as the hull collapsed so it’s not exactly what the OP is looking for.

This show was more about glorifying Ballard (deservedly IMO) than talking about science, but it had some great stories that might feed the OP’s curiosity.

7

For info about human limitations, have a look at some records set by deep sea pearl divers.

8

Don’t those records involve people diving with regular sea-level-pressure air in their lungs, without even scuba equipment? Or am I thinking of something else?

9

Here’s a link to the current freediving records. The current maximum is 162 meters.

10

Well, do keep in mind that while the change in density may be minimal, the pressure will change you to the same density as the surrounding water. Humans prefer being considerably less dense than seawater in the first place, even at sea level, which is why most of us float depending on how much air is in your lungs.

I’d imagine that even if your lungs could be magically protected, your circulatory system would fail as all the blood vessels would be closed by the high pressure. Again, that is a guess though.

11

According to this site water only compresses 1% a mile down. So all of the water in you would compress by about 1%. Your airspaces (ears, lungs, sinuses, maybe intestinal gases) would compress a lot more. But if you could magically protect them - like by replacing with a breathable liquid - you should be able to survive a mile down. The pressure wouldn’t collapse your blood vessels - they are filled with water (basically) as well.

“Crush depth” is a term usually applied to submarines or sinking, metal hulled ships. It means the depth at which the hull is no longer capable of supporting the pressure differential between the surrounding water and the 1 atm pressure inside. It doesn’t really apply to humans - if you were sinking to the bottom your lungs and other airspaces would fill with water as you drowned, but your body would remain pretty much intact - at least until the crabs arrived :wink:

12

As a tangent:
Why does pressure increase with depth?

It might be something really obvious, but I can’t see that it’s just a matter of accumulating more mass… hmm…
I’ll try again. If I carry 100 lbs, gravity will make me feel like there is a certain amount of pressure. Add another 100lbs and the pressure doubles. But I have a hard time understanding how that would accumulate under water. Obviously, it does, but why is that so?

13

One of my local PBS stations rebroadcast the Mysteries of the Deep episode of Scientific American Frontiers last week. It was fascinating. There’s a website for the episode with a bunch of clips that you can watch online. There’s also a transcript available, but it’s not as interesting as seeing the pictures taken from the deep ocean.

14

I’m not terribly good at math, so if I screw this up, someone correct me.

The answer is that water has weight. The same way air has weight, but more of it. One liter of water weighs 1 kilogram and takes up one cubic decimeter of space, so if you’re one kilometer down, one square decimeter of area (15.5 square inches) will have 1000 liters (1000 kilograms=2200 pounds) of water sitting on top of it.

15

Actually, I think that should be 10,000 liters of water (10,000 kilograms=22,000 pounds) sitting on that one square decimeter.

16

So is the fiction where the people use high-pressure breathable gas to fill their deep-sea habitats actually realistic? Is there any reason why you can’t compensate for external pressure by increasing the internal atmospheric pressure? Shouldn’t you be able to compensate to the point where a rupture in the sea habitat wouldn’t even be explosive, just by pumping in helium (or whatever) until the pressure equalizes?

17

If I were to send a bottle down to the bottom of the ocean (suppose it’s made of some extremely pressure-resistant acrylic or something) and somehow get it filled up with a sample of the water and then sealed, and then brought it to the top, what would happen to it? Would it freeze on the way up? If I opened it up and poured it on my hands, would it burn them (burned from extreme cold, that is)?

18

As soon as the water entered the bottle, it would either be at the surface pressure (if you were able to maintain that within the bottle) or it would remain at the pressure of the depths, which would mean that as the bottle returned to the surface, the water would expand back that 1% noted by muldoonthief.

Now, if it was already at surface pressure when you got it it back and opened it, it would just be cold (the 4ºC mentioned earlier). If, however, it was still under pressure, it would then expend rapidly upon openning, and in that case I would think it would heat up in the expansion.

ISTR a thing about people being incinerated by the air compressing and heating ahead of the onrush of water when a sub implodes, but I don’t have a clue where that memory of data is coming from.

19

I should have noted that the above is just my SWAG.

20

A 1% expansion would result in very little heating (adiabatic heating?). Besides, depending on speed with which you brought up the bottle, and the insulation on the bottle, it would have probably heated up a bit on the way just from conduction.

I read the stuff about people incinerating in a sub in Tom Clancy’s Red Storm Rising - his World War 3 book. IIRC, the helicopter pilot who said it didn’t know if it was true or not - sounds like a military UL that Clancy picked up.