Constructed using light steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 nets, indicator nets were often anchored at various depths to the sea bed around Allied

Allies of World War II

The Allies of World War II were the countries that opposed the Axis powers during the Second World War . Former Axis states contributing to the Allied victory are not considered Allied states...

 naval base

Naval base

A naval base is a military base, where warships and naval ships are deployed when they have no mission at sea or want to restock. Usually ships may also perform some minor repairs. Some naval bases are temporary homes to aircraft that usually stay on the ships but are undergoing maintenance while...

s during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. They were intended to entangle U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

 traffic of the enemy, even though the submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s often managed to disentangle themselves and escape before being blown up by depth charge

Depth charge

A depth charge is an anti-submarine warfare weapon intended to destroy or cripple a target submarine by the shock of exploding near it. Most use explosives and a fuze set to go off at a preselected depth in the ocean. Depth charges can be dropped by either surface ships, patrol aircraft, or from...

s.

Predominantly deployed by the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 as a means of discouraging enemy submarines from entering Allied waters, indicator nets were used extensively during the war. Individual nets were sometimes as much as 100 meters in length.

Instead of being used as the sole anti-submarine measure, indicator nets were often mixed with extensive minefields

Land mine

A land mine is usually a weight-triggered explosive device which is intended to damage a target—either human or inanimate—by means of a blast and/or fragment impact....

 and patrolling warship

Warship

A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...

s. Sometimes mines

Naval mine

A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...

 were attached directly to the nets, thus reducing submarine survival chances.

After a submarine became entangled in the net, a marker buoy

Buoy

A buoy is a floating device that can have many different purposes. It can be anchored or allowed to drift. The word, of Old French or Middle Dutch origin, is now most commonly in UK English, although some orthoepists have traditionally prescribed the pronunciation...

 attached to the net drifted along the water's surface indicating an enemy below. The first example of indicator nets causing the destruction of a U-boat occurred at Dover

Dover

Dover is a town and major ferry port in the home county of Kent, in South East England. It faces France across the narrowest part of the English Channel, and lies south-east of Canterbury; east of Kent's administrative capital Maidstone; and north-east along the coastline from Dungeness and Hastings...

 when the U-8

SM U-8

SM U-8 was one of the 329 submarines serving in the Imperial German Navy in World War I.U-8 was engaged in the naval warfare and took part in the First Battle of the Atlantic....

became entangled on March 4, 1915.