Eli5: How do the dig really deep holes that arent wide enough to take an excavator into the hole?

Question

Theres a scuba diving center I drive by while working that started building a pool to practice in. The pool is only about a 20ft x 20ft square however it has to be at least 20 ft deep (im guessing, but if its to practice scuba diving its pretty deep.)

I was wondering how they dig a hole that deep when the hole is not wide enough to take an excavator into like im used to seeing.

In: 0

Answer 1

20ft down is doable with a large excavator.
here is an excavator digging a deep and narrow well:

if they need to, they might made the hole wider on top, to give excavator the angle it needs.