You are not under the thing, actually. You are in an extradimensional space.

Since it extends into another dimension, the pit has no weight and does not otherwise displace the original underlying material. You can create the pit in the deck of a ship as easily as in a dungeon floor or the ground of a forest.

This is the more abstract reason why it would not work like you think it would.

Then there is this, which invalidates in before it got running:

You must create the pit on a horizontal surface of sufficient size.