used where anchoring is prohibited or impracticable. Decompression stations When a significant amount of decompression is planned by several divers at a site where they can reasonably expect to surface on the shotline, they may consider deploying a decompression trapeze. This keeps the divers together where it is easier to monitor them and provide support for the last part of the dive. A decompression trapeze can also be dropped by the boat for a group of divers by pre-arranged signal, such as a second decompression buoy at the group. Drift diving Drift diving occurs when the divers allow themselves to be conveyed past the bottom by the prevailing current, minimising the effort required to cover a relatively large distance, but moving the divers away from the start position and making their surfacing position uncertain. The usual way of monitoring the position of a group is by the lead diver towing a surface marker buoy, and the divers of the group all following the dive leader. The dive boat follows the buoy at a distance where the lookouts can see if any diver surfaces near the buoy. Divers from the group either surface along the buoy line, or if there is vertical current shear or they are separated from the group, deploy their own DSMB and surface on that. Groups of inexperienced divers may be escorted by a second divemaster who tries to keep them together, directs stragglers back to the dive leader, and keeps track of remaining gas. Divers may be formally grouped in buddy pairs, surfacing together when the first is low on gas, or more informally, and rely on the divemasters if they have a problem. Wreck diving A major consideration of wreck diving is finding the wreck and getting the divers to the wreck with the minimum risk of getting lost. There are several methods for getting the divers to the wreck. The preferred method will depend on local conditions. In low visibility hooking onto the wreck is a reliable way of ensuring the divers will find it, but this procedure requires a wreck that is structurally suitable for snagging with a grapnel or anchor. A shotline which can be dropped off the wreckage is less likely to damage the wreck or become snagged and difficult to retrieve, but this requires appropriate visibility for the divers to be sure of finding the wreckage. When it is important