The Tardigrade is tiny, and basically indestructible. Pound for pound that makes it officially well hard as a minimum. Frankly it would smirk at a nuclear bomb, and just carry on doing what it does.
The Tardigrade
Can live in boiling water, solid ice, can survive a decade without water, deepest trenches of the sea, and in space when subject to cosmic radiation. Has dagger-like teeth. Here before us, and will be here long after us.