The active site cavity in acetylcholinesterase. The use of the haptic device to study the deep active site cavity in acetylcholinesterase is illustrated. The probe is in the cavity as indicated by the 2D cursor (depicted as a circle with cross-wires) that gives the probe's x-y position when it is occluded. This allows the user to view the whole molecule whilst gaining an appreciation of the shape and extent of the cavity through their sense of touch by moving the probe around. Cystallographic water molecules are depicted as semi-opaque spheres in cyan. These are seen but not felt, thus the term "ghost water".