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Imagine an infinite empty space except for a local mass distribution resembling our observable universe. I don't think there lies no mass behind the horizon but let's imagine that this is the case. The event horizon lies outside of this local universe and it might be clear that black holes are present in this universe. Can we say that black holes can reside in black holes?

Qmechanic
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MatterGauge
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There are solutions in General Relativity in which you can cross more than you event horizon and perhaps find a lot of structure beyond the horizon. For example, the well known Reissner–Nordström and Kerr black holes have a "tower" of event horizons. After you cross the first one, you still have infinitely many event horizons you can keep crossing. This can be seen from their Penrose diagram and is very well exemplified in PBS Space Time's video Mapping the Multiverse.

I should point out, however, that these "inner horizons" concern the maximal extensions of these spacetimes and there are physicists who believe this mathematical procedure to be unphysical. Hence, I believe an adequate answer for your question would be:

General Relativity does allow for these sorts of solutions, but since we have no way of probing inside a black hole's event horizon we can't really tell whether they are physical or not.

Níck Aguiar Alves
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