Sinking on purpose
A SUBMARINE works by changing its own weight. Between the inner and outer HULL sit BALLAST tanks: open the vents and seawater floods in until the VESSEL weighs more than the water it displaces, and down it goes. Blow the water back out with compressed air and it rises again. Fish manage the same trick with a swim bladder, adding or removing gas to hang at a chosen DEPTH without swimming.
Fine adjustments come from hydroplanes, small wings on the hull that tilt to push the nose up or down as the boat moves forward. A submarine under way is flying through water more than floating in it.
Listening rather than looking
Below a few hundred metres there is no daylight at all, so submarines find their way by sound. Everyone knows the ping of active SONAR — send out a pulse, time the echo — but a naval boat almost never uses it, because that ping tells every listener for miles exactly where you are.
Nearly all the work is passive: hydrophones simply listening, and sonar operators trained to recognise a ship's type from the beat of its PROPELLER. That is why submarines are built obsessively quiet, with machinery mounted on rubber blocks and hulls clad in sound-absorbing tiles. The best boats are harder to hear than the background noise of the sea itself.
Three months without surfacing
A NUCLEAR submarine never stops for fuel or fresh air. Its reactor runs for years, seawater is distilled into drinking water, oxygen is made by splitting seawater with electricity, and the carbon dioxide the CREW breathes out is scrubbed chemically from the air.
What limits a patrol is food and people. Boats typically stay under for two or three months, because that is how long the stores last and how long a crew can reasonably live in a sealed tube with no daylight, sharing bunks in shifts. Families can send a few lines of text; almost nothing goes the other way, because transmitting would give the boat away.
Radio does not go through water
Ordinary radio stops dead a few metres below the surface, which leaves navies with an awkward problem: how do you reach a boat hiding hundreds of metres down without making it come up and give itself away?
The answer is very long radio waves, which creep further into seawater — and the longer the wave, the bigger the transmitter. Shore stations built for the job string antennas across several kilometres of countryside and consume enormous power, all to send a message at a few characters a minute. A submarine can also trail a floating wire behind it, or raise a slim mast for a burst of satellite traffic, but every option means coming shallower and risking being seen.
The deepest dive ever made
In January 1960 Jacques Piccard and Don Walsh rode a steel sphere called Trieste to the bottom of the Mariana Trench, nearly eleven kilometres down, where the PRESSURE is over a thousand times what it is at the surface. A window cracked on the way down. They carried on, spent twenty minutes on the sea floor, and reported seeing a flatfish — a claim biologists argued about for decades afterwards.
There is a neat family thread here. Jacques's father Auguste designed that craft, and also the balloon in which he became the first person to reach the stratosphere. Jacques's son Bertrand later flew the first balloon non-stop around the world, which is a story our hot air balloons puzzle tells.