to get back to the shotline for ascent, it may be tied to the wreckage by the first divers on site using a guide-line, which is retrieved by the last divers to leave. When there is a strong current, it may be necessary to drop in from up-current, a technique sometimes known as "parachuting in" or "free drop". The wreck may be first marked with a shotline, if this is considered useful or necessary. Divers may surface on the shotline, anchorline or personal DSMB depending on the conditions. When there are alternative methods for descent, including free drop and descent on the shotline. An (or Jersey upline) may be used to limit drift during ascent. This is a fairly substantial natural fibre rope which is deployed from the bottom using a small lift bag to provide the equivalent of a shotline. The lower end is tied off to the bottom, usually on a wreck, and the diver ascends on the line to avoid being swept away from the site by currents. After reaching the surface, the last diver cuts the line and it sinks back down, Natural fibre is used so the line rots away over a few years. Towing a diver Occasionally a diver may be towed using a "sled", an unpowered device towed behind a surface vessel which conserves the diver's energy and allows more distance to be covered for a given air consumption and bottom time. The depth is usually controlled by the diver by using diving planes or by tilting the whole sled. Some sleds are faired to reduce drag on the diver. Emergency procedures Diver recall There are several ways to recall divers to the surface during a dive. Some are not universally known or immediately obvious, and the chances of them being understood correctly and responded to appropriately are increased if they are specified in the dive briefing. Pyrotechnic diver recall devices (Thunderflash) are small explosive devices calibrated to make sufficient noise to alert any divers in the vicinity without significant risk of injury to diver or environment. They have the advantages of being small, compact, and not easily confused with anything else likely to occur during the dive. They have disadvantages of a limited shelf life, and must be transported, stored, used and disposed of according to manufacturer's instructions and legal constraints. A surface marker buoy line may be used to signal to