How dense is the water at the bottom of the Mariana Trench?

I'm going to approach this differently from the other answers by appealing to the concept of bulk modulus, which measures volumetric change with pressure. read more

Thus, calculating the density change for the water pressure at the Mariana Trench is now simply a matter of plugging and chugging into Eq. (3) if the pressure difference dp is small compared with K. For if this condition holds we can exploit Eq. read more

Owing to the very high pressure exerted by the water column above, the density of water at the Mariana Trench increases by about 5 percent, which means that 95 liters of water at the bottom of trench will have the mass equal to 100 liters of water at the surface. read more

At the bottom of the trench the water column above exerts a pressure of 1,086 bars (15,750 psi), more than 1,000 times the standard atmospheric pressure at sea level. At this pressure, the density of water is increased by 4.96%, so that 95 litres of water under the pressure of the Challenger Deep would contain the same mass as 100 litres at the surface. read more