Can there be a black hole inside the Earth right now & would we notice it?
Well, technically…
But first things first.
A black hole with an event horizon that is as large in geometric size as the crust of the Earth would weigh more than 2,000 times as much as the Sun (more than 700 million Earths). Needless to say, its presence would be, shall we say, a tad destructive.
A black hole that weighs as much as the Earth, on the other hand, has a radius of fewer than 9 millimeters. But no, no such black hole could lurk deep inside the Earth; its mass would be highly anomalous, and it would consume material from the Earth, over time “eating” the whole planet (and long before then, disrupting the planet in various unpleasant ways.)
But what about a much smaller black hole? Say, a black hole with a lifetime of 10 billion years. That black hole is “tiny”, relatively speaking: its mass would be about 150 million metric tons, but it would be smaller in size than a proton. It would also be tremendously hot, emitting about 15 gigawatts in the form of mainly gamma rays, with a surface temperature of nearly 800 billion degrees kelvin.
But because it is so tiny, it would not “eat” the Earth. Such a tiny black hole is a very inefficient eater. So if such a black hole was sitting deep inside the Earth without moving about, it would probably remain quite unnoticeable. It would be a powerful point source of heat, so stuff in its immediate vicinity would become a very hot, dense plasma, but I think that’s about it… its influence would pretty much cease beyond a few ten meters.
Trouble is, there is no known mechanism to produce such a tiny black hole. The only mechanism that we know about is stellar collapse, and the smallest black hole that stellar collapse can produce weighs about three Suns or a million Earths.
So on that basis, I’d say that it is extremely unlikely that a black hole exists deep inside the Earth, even if theoretically, the possibility cannot be completely excluded.