I'm thinking an ounce of metallic hydrogen suddenly pops into existence in a high enough density that a thermonuclear explosion occurs.
Why wouldn't that implode ?
From my basic understanding of fusion reactions, is that if enough hydrogen is in a small enough spot in a high enough energy state, they'll collide and fuse together. it may only be a couple atoms or so fusing together after some sort of process which I can't really describe or really know about at the current moment.
even if a thermonuclear reaction didnt occur and instead imploded, it could be that enough metallic hydrogen comes into existence that when it violently returns back to gaseous form it also catches fire, resulting in a plain old regular detonation.
Also, a bizarre magical Item
The Mini BazaarA small pocket sized set of chalk, if a door is drawn onto a wallway, no bigger then a two thirds of a meter in height, a dimensional doorway to the Mini Bazaar is made. It is a place where miniature items that are typically sold in a Bazaar are sold. Chief among this are various spices, goods, curved swords and bone flutes. No no way, shape or form, is the user shrunk to be able to access the door, or do the inhabitants accept big currency, however the local gold craftsman will accept big currency of metal coins in return for the local(?) currency of the area, obsidian pieces.