“Wait! Water is incompressible, right?”

Well, to be exact, water isn’t very compressible, but squeeze it hard enough and it will yield. The at 0° C is 5.1\times 10^{-10}~{\rm Pa}^{-1}, so at 4 km in the deep ocean (a little deeper than the wreck of the Titanic at 3800 metres), water is compressed around 2% by volume. At the bottom of the Mariana Trench at around 20 km, the water, under a pressure of 1000 bars (times standard atmospheric pressure) is about 5% denser than at the surface.

But why stop there? With a 150 tonne hydraulic press driving a piston to amplify the force, it’s possible to achieve a pressure of 2000 bars, twice that of the deepest point in the oceans. Units Calculator computes compression under such pressure as:

What will happen to ordinary objects under such pressure? Scuba tanks are typically pressurised to between 200 and 300 bars, so it would be amusing see if one, fully filled with air, was crushed by this pressure, but sadly the test cell is insufficiently large. I haven’t been able to find the pressure specification for those 8 gram carbon dioxide cartridges used in soda siphons: it would be interesting to subject one to 2000 bars.