Enter that room and you breached a Schwarzschild radius of something not canny, and that cephalopod corpse was the singularity.
-- Kraken, China Mieville.
The Schwarzchild radius is the radius of the hypothetical sphere of matter within which you would have to condense that matter sufficiently to create a gravitational field in which the escape velocity is the speed of light; in other words, it's the size of a black hole for any given amount of matter.
The Schwarzchild radius for the sun is 3 km; condense the sun to that size, and you have a black hole. For Earth, and this is fascinating to me, the radius is 9 mm. The Schwarzchild radius for me is 1.738 x 10-25 m.
Currently, the smallest black hole known to man is 24km in diameter. Scientists theorize there may be a far smaller one, but Donald Trump wouldn't release his medical records.