How quickly can you dig a pit trap hole with Mold Earth?

Question

The hole for "the simplest of pit traps" consists of "a 10-foot-deep hole" in the ground (Xanathar's Guide to Everything p.114, and Dungeon Master's Guide p.122) because "at the end of a fall, you take 1d6 bludgeoning damage for every 10 feet you fell" (XGtE p.77, and Player's Handbook p.183).

Assuming the ground in question meets the requirements for "loose earth" (but isn't so loose it immediately subsides back into the hole), as an Action the mold earth cantrip can dig a 5-foot-deep hole:

You choose a portion of dirt or stone that you can see within range and that fits within a 5-foot cube. You manipulate it in one of the following ways:

• If you target an area of loose earth, you can instantaneously excavate it, move it along the ground, and deposit it up to 5 feet away. ...

Can you dig a 10-foot-deep hole in two actions with two castings of mold earth, or does the depth matter when you try to excavate the second 5-foot cube so you would need to dig an extra step first to move it to?

Answer 1

It takes 3 actions.

The depth matters, but that doesn't prevent you from digging a 10-foot pit. It just requires extra steps:

1. Excavate a 5-foot pit (Pit 1) by moving the earth up-and-east.
2. Excavate another 5-foot pit (Pit 2) north of Pit 1 by moving the earth up-and-west.
3. Excavate a 5-foot pit under Pit 1 by moving the earth up-and-north. This fills in Pit 2, and deepens Pit 1 to 10 feet.

Answer 2

It's not clear, so up to the DM

The 5e rules (as many others in the D&D pedigree) are notoriously vague when it comes to height or vertical space. For example, the space a creature controls is also defined only by floor area of feet, nothing about height.

The ground (especially in relation to move Earth) is likewise not well defined. Is the ground just the horizontal surface? Not clear. If it is, then moving it to the adjacent space is 5 feet away on the ground.

So, the rules do not conclusively settle this and it will be up to your DM to interpret it. Tasha's Cauldron of Everything, p. 4:

The rules of D&D cover many of the twists and turns that come up in play, but the possibilities are so vast that the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed, aiming for a course that brings the most enjoyment to your whole group.

I would likely rule (and so does our DM) that moving it up the wall counts as movement along the ground. Otherwise, if vertical distance does not count, you will be able to dig holes thousands of feet deep, at a rate of 50 feet per minute.

If you allow it, a 10 foot hole can be accomplished in just two actions. Assuming someone occupies the space they would only fall 5 feet each time, and take no damage, but may have difficulty if getting out.