Helium.
I had an interesting dream recently. A band of cohorts and myself were devising a plan to make certain that at a specific moment in time a nuclear weapon that had already been launched would collide with a rock and thus render it useless. We managed to mount the nuclear weapon onto the top of a speeding train. We attached balloons to the top of the weapon to keep it afloat and at the desired altitude should the train route fail. To our dismay, the nuclear weapon began to climb higher in the sky as a result of all the balloons we had attached to it. I climbed onto the nuclear weapon and begain to pop some of the balloons as we sped toward the planned collision point.