I've done my own head in - all because I didn't study basic gas physics hard enough. So... Assume I have an indestructible, impermeable, rigid globe (of a fictional metal, say) containing a complete vacuum, and I am able to change the size of the sphere at will. My globe and I are standing at sea level. If I keep increasing the size - and so, volume - of the globe, will the globe eventually rise upwards into the atmosphere? It will, right? I guess the fact that the pressure inside the globe is always zero is kind of throwing me, for some reason.