A silt out is a situation when underwater visibility is rapidly reduced to zero.

It often happens from divers' swimfin

Swimfin

Swimfins, swim fins, fins or flippers are worn on the foot or leg and made from finlike rubber or plastic, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, kneeboarding, riverboarding, and various types of underwater diving.Scuba divers use...

s disturbing silt, particularly in caves or in still fresh water.

Another common cause when penetrating wrecks is bubbles from scuba gear

Scuba set

A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....

 making rust fall down from above. The inside of wrecks or caves are often covered in a fine sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 which might get stirred up accidentally by the diver's motions, causing a silt out.

Silt outs are dangerous situations and courses in overhead environment diving, such as wreck

Wreck diving

Wreck diving is a type of recreational diving where shipwrecks are explored. Although most wreck dive sites are at shipwrecks, there is an increasing trend to scuttle retired ships to create artificial reef sites...

 or cave

Cave diving

Cave diving is a type of technical diving in which specialized equipment is used to enable the exploration of caves which are at least partially filled with water. In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common...

 diving teaches various methods to cope with zero visibility.