While I don’t know the depths at which the Argonaut rides, let’s assume that it takes on enough air for it to be neutrally buoyant at 10 metres, i.e., 2 bar absolute pressure. Since the pressure at the surface is 1 bar absolute pressure, the air inside the shell is compressed to one half of its volume at 10 m. The absolute change in buoyancy depends on the change of the volume of air, so let’s assume that it takes on 16 millilitres of air, ammounting to 16 grams of buoyancy at the surface (you’ll see why such an “odd” number in a moment).

If it’s neutrally buoyant at 10 metres, where the air is compressed to 8 millilitres, at the surface its shell contains 16 mililitres of air, giving it 8 grams of extra buoyancy compared to the 8 mililitres at 10 metres. That means it will have to provide 8 grams of vertical motive force at the surface to commence the dive.

Now let’s assume that it becomes distracted by a beautiful Argonaut of the opposite sex, forgets to swim, and sinks to 30 metres. At that depth the absolute pressure is 4 bar and the volume of air is now 4 mililitres, which ammounts to 4 grams of negative buoyancy (a reduction from 8 to 4 millilitres of air). Since we know that it can produce 8 grams of vertical motive force to dive, it can easily provide half that ammount to be able to swim back from 30 to 10 metres.

If it sinks to 70 metres, the absolute pressure is 8 bar, the volume reduces to 2 mililitres, and it has to provide 6 grams of force – again not a problem. If it sinks to 150 metres, the absolute pressure is 16 bar, the volume reduces to 1 millilitre and it has to provide 7 grams of vertical force – again not a problem. In fact, it can descend to the bottom of the Mariana Trench and still only has to provide at most 8 grams of vertical force to enable it to rise to the surface again, because in the limit of infinite pressure, the air is compressed to zero millilitres, thus making it 8 grams negatively buoyant.

An if you assume it normally rides at a depth of 30 metres, 4 bars of absolute pressure, the force needed to ascend from any depth deeper than 30 metres is at most one quarter of the force it needs to descend to 30 metres.

While I don’t know the details of balast tanks in a submarine, I suspect the real problem there is the rate of change of buoyancy. There’s a limit to how fast the water can enter and leave the tanks through the holes in the hull but, more importantly, how quickly the air can be pumped into the tanks through the pipes. What I think happens is that after a certain change of depth you simply cannot pump air quickly enough into the tanks to offset the input of water and the bubble of air in the tanks keeps getting smaller and smaller.

There’s a similar situation in scuba diving. When you ascend, you have to vent air from the BCD to keep the volume of air inside the BCD more or less constant, compensating only for the increasing volume of the neoprene and the slowly decreasing mass of air in the tank – a difference of around 4 kilograms from the start to the end of the dive for a 15 litre tank pumped to 200 bar.

If you’re do not start venting soon enough after the start of ascent, you get into the situation where the outflow of air from the BCD is too low to account for the increasing volume caused by the decreasing pressure and you get a so-called runaway ascent, a Very Bad Thing that can lead to DCI. Exactly the same situation occurs with a drysuit, that contains an even larger ammount of air, and where it’s even more crucial to keep it balanced. A drysuit diver in an uncontrolled ascent can breach the surface of the water almost like a jumping Great White shark.

OK a submarine can be essentially thought of as an inverted jam jar…. and with a certain amount of mass, with a certain volume of air inside it, it will descend to a certain depth and “float” there’.

However if the mass increases or the jar is forced to go deeper, the pressure causes the air inside the inverted jam jar to decrease in volume, thus decreasing it’s displacement, thereby lowering it’s boyancy.

It’s the same reason that submarines can get into a situation that if they dive really fast to depth or some other situation occurs – they get into a position that no matter how much air they blow into the tanks to increase the volume of air and thus increase the boyancy of the craft – that the air keeps getting compressed into a smaller and smaller volume – and the sub in certain situations such as loss of motive power etc., will continue to sink until it implodes or hits bottom.

This is what the Agonaut is doing when it descends down past neutral boyancy – and it rides at depth – and I assume that it mostly operates within a narrow band of negative to neutral boyancy most of the time.

I do not know enough about the animal, but I would speculate that it probably could expand the airbubble in a diaphramatic way, to change the level or depth of boyancy, where neutrality occours, and that it may not technically be operating at a negative boyancy, in the region where negative boyancy commences.

1) The shell walls are quite thin, less than a mm thick and easily broken. They are much thinner and weaker than those of the true nautilus.

2) Despite the resemblance, the shell differs from that of nautilioids and all other molluscs both in terms of structure (it is calcitic rather than aragonitic) and is secreted not by the mantle as in other molluscs but by specialized organs on the arm. For these reasons it is regarded as an evolutionary novelty and not a reversal. The shell is not homologous with the ancestral molluscan shell as are the internal and external shells of other cephalopods.

3) The shell of the Argonaut is only secreted by mature (fully grown) females and the females can survive outside the shell.

The evolutionary origins of the Argonaut shell have inspired quite a lot of, often fanciful, speculation but so far as I know is still a complete mystery (perhaps Finn has some more secrets to reveal about this though!)

Before diving the spermaceti is cooled with water, solidifying it, thus increasing density and decreasing buoyancy, making it easier to dive. During the dive the substance slowly heats up, decreasing density and increasing buoyancy, making it easier to surface.

An infant in a womb does not breathe – his blood is oxygenated through the umbilical and there is no circulation of blood through the lungs. However, it’s the infant’s heart that pumps the blood around and to enable the blood to pass from the venous to the arterial side, a hole exists between the two sides. After the birth this hole usually closes. However, in a certain percentage of adults the hole never closes.

If a diver ascends too quickly, the dissolved nitrogen in the tissues and blood forms bubbles. The bubbles in tissues cause the bends, or DCI (decompression illness). The bubbles in the blood, however, are trapped in the arterioles of the lungs and gas then diffuses out into the lungs and is breathed out. But if the hole is not closed, some of these bubbles pass directly into the arterial part and cause the arterial blood embolism.

The other major cause is pulmonary barotrauma caused by holding breath while ascending after breathing compressed gas. In essence the overpressure of the gas in the lungs literaly tears the lungs open from the inside out. This can have several effects, among them pneumothorax (a collapse of the lung), air under your skin (you literally get a bubble under your skin, ususally in the neck area) and again arterial gas embolism, where the gas is forced through the torn venoles in the lungs into the arterial circulation and from there into the tissues and, most importantly, the brain. And this is the greatest danger – parts of brain get cut off from circulation and you have very serious problems. It takes a surprisingly small overpressure to do damage: breathing air at just 1.2m below surface and holding your breath can be enough.

A good place to read about these things is http://scuba-doc.com/

Concerning the buoyancy of divers: The neoprene divesuits compress as you go deeper and therefore become less buoyant. In the old days, before the BCDs (buoyancy control devices – essentialy air bladders that you war in a form a jacket and that can be filled with air from a scuba tank and emptied into the water), the divers would weigh themselves with the ammount of lead that would depend on their planned depth. Usually they had to swim down to about half the planned depth and the rest was just letting the gravity do its work. On the way up they again had to swim to about half way up and then the buoyancy would complete the job. Nowadays BCDs are used to compensate the changes of buoyancy and maintain neutral buoyancy at any depth.

“For each atmosphere increase in pressure, the volume of water would decrease 46.4 parts per million.”

Oh, and this might be confusing: “In the ocean or other large body of water, this pressure increases [~ 1 atm] every 10m (33ft).”

This is why divers have to make sure to open their eustachian tube and regulate depth change rates, or the pressure differential would eventually be harmful, with gas embolism occurring (“the bends”) et cetera.