I had a peek at the source code and although I don’t actually know Rust it looks like that error comes from a check for character length in the function “is_valid_body_field”. Strangely it does the same check twice against two variables “POST_BODY_MAX_LENGTH” and “BODY_MAX_LENGTH”.
The smaller of the two is BODY_MAX_LENGTH which is set at 10000, so I assume the max character limit is 10,000. There are no other checks in that function other than the character count and that’s the only place in the source code that the text “invalid_body_field” shows up so I assume it’s only sent as a response to too much text, but as I said I don’t actually know Rust so I could be wrong.
I think the phrase down the rabbit hole is actually referring to Alice in Wonderland. But.
I would say the organism that tends to burrow the deepest into the Earth is humans. Average oil well depth appears to be around 5,964 feet (1818 meters), that’s pretty deep. The deepest hole we ever drilled is supposedly the Kola Superdeep Borehole dug by the Soviets, it was 40,230 feet (12.2km) deep.
Perhaps not answer your looking for though.