Dave Loeffler in Sirens Passage, the first horizontal passage one encounters when one enters the Underworld after the six pitches of the Gaffered to the Walls series. Notice the mud floor, which consists of a very thin dark layer covering a thicker band of light brown clay; it is a conservationist's nightmare, impossible to walk across without destroying. The inevitable accumulation of mud on one's gear also renders the prusik out very interesting as the rope gradually turns into a cylindrical stick of mud!
Photo © Earl Merson, 2004