Scientists have found a way to reduce density by increasing pressure. Source
This new discovery is entirely counterintuitive, so much so that even the researchers who discovered it didn't believe it for a long time. By putting zinc cyanide in a diamond-anvil cell and surrounding it with a variety of fluids, they were able to apply immense pressures (0.9 to 1.8 gigapascals, otherwise known as 'a lot of pressure') and make the zinc cyanide actually decrease in density. They observed that, under these conditions, the compound became porous, much like a sponge, and expanded. This flies in the face of everything that we are taught to believe. After all, don't things get smaller and harder when you apply pressure?
Now, you might ask: what's the use of all this? Well, porous materials are used for everything from water filtration to medical instruments. By making previously non-porous materials into porous materials, these scientists have opened the way for countless new innovations that will save lives and generally make the world a better place.
For more information, check out this article.