Venus, and how we could colonize it.
Basically, Venus has a very dense atmosphere. If you were to take a giant baloon filled with oxygen and shove it near Venus, it would float in the atmopshere, assuming it didn't explode due to the extreme temperatures. You could test this by filling a sealed box with a gas heavier than oxygen but leave the top off. Because the gas is heavier than oxygen it won't escape out of the top. Then take a light bag, fill it with air, seal it and put a test mouse in it. Place the bag with the mouse in it inside the box and it should float. Bam, we're one step closer to colonizing a planet.
Be careful though, gasses heavier than oxygen at room temperature tend to be lethal.