Some bulletproof safes are sturdy enough to handle grenade impacts. Could we theoretically make an analogous atomic-proof safe that will handle something like an Hydrogen Bomb? It doesn't matter the shape of it or if it has a door or handles, the materials use to build it or if it has air on the inside. As long as we can cover the whole device and prevent any significant mechanical damage to be done on the outside, I'll consider it good enough for the task.
EDIT: I'm so sorry for not being clear enough, and I've seen really great answers to this question, some of which seems to me to actually solve this problem. What I mean is: what would happen if the bomb got detonated inside this huge safe. Like, it seems clear to me that a lead safe with the size of the earth with the radius of the planet as it's thickness would be more than enough to protect someone at the surface from the blast. So, with that said, what would be the minimum necessary to muffle the explosion?