It often happens from divers' swimfins disturbing silt, particularly in caves or in still fresh water.
Another common cause when penetrating wrecks is bubbles from scuba gear making rust fall down from above. The inside of wrecks are often covered in a fine sediment which might get stirred up accidentally by the diver's motions, causing a silt out.
Silt outs are dangerous situations and courses in wreck diving teaches various methods to cope with zero visibility. Always using lines during penetration dives is the most important safety measure as it helps divers find their way out. Training for silted out situations include exercises in following lines with a blacked-out mask. First training is performed by walking on land and later swimming along lines in the water. During underwater swimming, instructors simulate various situations in order to train the diver to be able to handle them. Such situations may be regulator free-flowing, other lines hooking up to the divers' line, and new gas cylinders placed together with the original decompression cylinders which the diver is supposed to switch to during ascent: wreck penetration at this level is considered technical diving and divers are therefore often expected to know how to perform decompression dives. The final stages of wreck penetration training is held inside wrecks with blacked-out masks and the divers are supposed to find their way out using their guiding line.
Alternative methods of signalling, by squeezing the buddy's hand, are used instead of ordinary hand signs.