Underwater microphone. Because air is an easily compressible medium, but water is not; sound waves (pressure waves) travelling in water involve small displacements of the water molecules and high energies, whereas sound waves travelling in air involve large displacements of the air molecules but low energies.
Consequently, if you put an ordinary microphone in a box, the sound waves in the water won't move the walls of the box very much, won't compress the air in the box very much, and so won't move the diaphragm of the microphone very much.
The only solution is to have a specially designed microphone in which the diaphragm is actually in contact with the water and is sensitive to very small displacements.