This is not a joke. Some rocks can float in water for years at a time, and today scientists know how they do that and the reasons why they finally end up sinking.
X-ray studies have helped researchers resolve the mystery of the Pumice Stone, a very light and porous volcanic rock. The surprisingly long-lasting capacity of these rocks to float, even join together to form islands several kilometres long in the middle of the ocean, can help researchers discover underwater volcanic eruptions.
And, more than that, learning how they stay afloat can help us understand how living species are disseminated all over the planet: the pumice stone is, in effect, very rich in nutrients and serves as a means of maritime transport for many plants and other organisms.
Why does the gas stay trapped?
Despite the fact that scientists know that pumice stone floats on account of the pockets of gas contained in their pores, nobody has yet been able to explain how or why those gases stay trapped inside these rocks for such prolonged periods of time. If you soak a sponge with sufficient water it will end up sinking, but pumice rocks don’t.
Some pumice stones were even observed in a laboratory to sink in the evenings and bob up again during the day. Researchers used wax to coat and warm up small bits of pumice stone in water in an effort to understand what is making these rocks behave in this manner.
All of a mystery waiting to be resolved!