Negotiating the Mulu Sump requires some nerve. One definitely cannot allow panic to set in. It is a difficult blockage to pass, for although it averages nearly 20 feet wide and four feet high, the passage is filled with water to within three inches of the ceiling in many places. The trick is to peer ahead and locate the high spots. The whole experience is a hop-scotch from one high spot (which may only be big enough to get most of your face out of the water) to the next, ducking under and swimming. Fortunately, a near gale of air blows through the sump, which helps alleviate some sense of claustrophobia.