or wreck penetrations, a guide line is laid from a dive reel. In less critical conditions, many divers simply navigate by landmarks and memory, a procedure also known as pilotage or natural navigation. A scuba diver should always be aware of the remaining breathing gas supply, and the duration of diving time that this will safely support, taking into account the time required to surface safely and an allowance for foreseeable contingencies. This is usually monitored by using a submersible pressure gauge on each cylinder. Safety equipment A bailout cylinder provides emergency breathing gas sufficient for a safe emergency ascent. Any scuba diver who will be diving below a depth from which they are competent to do a safe emergency swimming ascent should ensure that they have an alternative breathing gas supply available at all times in case of a failure of the equipment they are breathing from at the time. Several systems are in common use depending on the planned dive profile. Most common, but least reliable, is relying on the dive buddy for gas sharing using a secondary second stage, commonly called an octopus regulator connected to the primary first stage. This system relies entirely on the dive buddy being immediately available to provide emergency gas. More reliable systems require the diver to carry an alternative gas supply sufficient to allow the diver to safely reach a place where more breathing gas is available. For open water recreational divers this is the surface. For technical divers it may be a stage cylinder positioned at a point on the exit path. An emergency gas supply must be sufficiently safe to breathe at any point on the planned dive profile at which it may be needed. This equipment may be a bailout cylinder, a bailout rebreather, a travel gas cylinder, or a decompression gas cylinder. When using a travel gas or decompression gas, the back gas (main gas supply) may be the designated emergency gas supply. Cutting tools such as knives, line cutters or shears are often carried by divers to cut loose from entanglement in nets or lines. A surface marker buoy (SMB) on a line held by the diver indicates the position of the diver to the surface personnel. This may be an inflatable marker deployed by the diver at the end of the dive, or a sealed float, towed for the whole dive. A surface marker also