> John Wheeler once calculated that if one took all the heavy water in all the oceans of the world, one could build a hydrogen bomb that would compress matter at the center so much that a black hole would be created
Possibly the largest artificial one, but pair instability supernova turn several solar masses of hydrogen all the way up into nickel-56 in a few seconds:
Edit: Just remembered that Isaac Arthur has spoken about artificially inducing a Type Ia supernova to synthesise heavy elements, and why sufficiently developed civilisations may want to do this at this scale.
> John Wheeler once calculated that if one took all the heavy water in all the oceans of the world, one could build a hydrogen bomb that would compress matter at the center so much that a black hole would be created