Your answer is in the notes at the beginning of level three.
Before the Axe of Unmaking awoke, level three was on average 70 feet beneath level two, making it 140 feet below ground level. As the text indicates, however, the warping effects of the Axe make it hard to put an exact "place" on where level three actually is...those (like dwarves) who can detect their depth underground get conflicting and sometimes contradictory senses of how deep in the earth they are.
This would be especially prevalent in the Well of the Axe itself. The spatial distortions would be out of control - if you chopped through the shaor crystal in the observatory on level one and jumped in, odds are you wouldn't fall 140 ft. It could be 5 feet, or it could be 28 miles.
Co-Author: The Almanac of the Endless Traders, DCC #13, DCC #29, DCC #49, DCC #51, DCC #52, DCC #63
Author: DCC #55: Isle of the Sea Drake, DCC #61: Citadel of the Corruptor, more to come....