Chand Baori is more than 100 feet deep and has 3500 steps. It was built in the 9th century, which is astounding to me -- I can't believe I'd never heard of it before I went there. (I knew vaguely about stepwells because I'd seen some photo essays on them.)
Even with the aid (presumably) of elephants, it's an amazing feat of engineering. After more than a thousand years, the stones are all still perfectly symmetrical.