From Pathfinder Beginner Box: Secrets of the Unlit Star, which isn't enabled.

Hidden Pit (BB)

HAZARD 0

Stealth +8 (or DC 0 Perception if the trapdoor is disabled or broken)

Description A wooden trapdoor covers a 10-foot square pit that's 20 feet deep.

Disable DC 12 Thievery to remove the trapdoor.

AC 10; Fort +1, Ref +1

Hardness 3, HP 12 (BT 6); Immunities critical hits

Pitfall

If a creature walks onto the trap door, that creature falls in unless it succeeds at a Reflex save to grab hold of the edge. The pit is 20 feet deep, dealing 10 bludgeoning damage to anyone who falls to the bottom.

Pathfinder Beginner Box: Secrets of the Unlit StarORC