Silt out

A silt out is a situation when underwater visibility is rapidly reduced to functional zero by disturbing fine particulate deposits on the bottom or other solid surfaces. This can happen in scuba and surface supplied diving, or in ROV and submersible operations, and is a more serious hazard for scuba diving in penetration situations where the route to the surface may be obscured.

The most common cause in scuba diving is when a divers' fins are used too forcefully or in the wrong direction; disturbing silt, particularly in caves, wrecks or in still fresh water environments. Specific non-silting underwater propulsion techniques are taught as standard on cave diving and technical-level wreck diving penetration courses; such as the Frog Kick, Modified Flutter Kick, Helicopter Turn, Pull-and-Glide, Finger Walking and the Back / Reverse Kick.

Another common cause when wreck diving is from exhaled bubbles from scuba gear disturbing overhead surfaces and making loose rust particles sink down from above. The inside of wrecks or caves are often covered in a fine sediment which might get stirred up accidentally by the diver's motions, causing a silt out.

Surface supplied divers have the additional problem of the umbilical trailing behind them, which is also likely to disturb silt where it makes contact, and the underwater work may also disturb silt. For these reasons surface supplied divers must often operate in very poor visibility.

Remotely operated underwater vehicles and submersibles manoeuver by using thrusters, and the water blast from thrusters has a similar effect to the wash from fins.

Silt outs are dangerous situations for scuba divers, particularly in enclosed spaces or when there is no direct access to the surface (an overhead environment). Training courses in overhead environment diving, such as wreck or cave diving teaches various methods to cope with zero visibility. Always using guidelines during penetration dives is an important safety measure as it helps divers find their way out.

...
Wikipedia