If you look hard enough, it is quite possible you will find a counterexample to any rule. One such rule is that as you apply pressure to something, it will become denser, but it turns out this is not always the case. Researchers at Argonne National Laboratory first discovered such a material years ago and since then have been testing it to convince them of what it was doing.
The material in question is zinc cyanide, which is used in electroplating. The researchers found that when they placed it in a diamond-anvil cell and applied pressures between 9000 and 18,000 times atmospheric pressure, it would spontaneously grow and develop sponge-like pores. This process also involves a fluid surrounding the material as it is squeezed, and it was discovered that different fluids affected the shapes of the pores. Two of the five fluids also caused the new, porous phases of the material to survive at normal pressures, instead of having its internal bonds rearrange again.
Porous materials have a variety of uses in different fields, including medicine and material separation, as the shape of the pores can determine what molecules it will store. This discovery could therefore greatly increase the number of porous framework materials that can be used by these different fields.
Source: Argonne National Laboratory