AVault has posted eight new screenshots from Gearbox Software's upcoming official add-on for Half-Life, Opposing Force, which show off an interesting sequence - here's a description:

As the player enters the room and heads for the apparent exit, an explosion erupts at the far end. This sets in motion a series of sequential triggers that collapses the room, piece by tiny piece, including rising slime, swinging ladders and crumbling bridges. The levelheaded gamer will be able to leap onto a falling bridge and escape the room at the last moment as the slime rises to the top. In the words of lead design Randy Pitchford, it is an “epic” sequence.