Sinking Floor (4e Hazard)
From D&D Wiki
A patch of ground that looks like damp, flat sand, sometimes littered with small debris. As soon as a Small or larger creature steps onto it, it liquefies and the creature begins to sink.
A patch of sinking floor is typically 2 squares by 2 squares in size. In an encounter, sinking floor is best used with artillery monsters, or skirmishers that can attack on the move: use them to strike at restrained players. Sinking floor is nasty when used with burrowing creatures such as the xorn (level 9 skirmisher, Monster Manual 2 p. 215.
When a creature sinks low enough, they cannot breathe: after each round after the 5th in this state they must pass a DC 20 Endurance check or suffocate.
Characters can use Athletics to jump over the sinking floor if they know where it is.