This resulted in basic technical flaws that, combined with the inadequate training of their operators, meant they never posed a real threat to Allied shipping, despite 324 submarines being delivered.

Originally proposed by Kapitänleutnant Bartels in 1942, but not acted on until the need for coastal defence in the event of an invasion became paramount.

[6] The initial prototype, officially titled Bunteboot (but better known as Adam), was heavily influenced by the British Welman submarine.

[6] The hull was built in three sections composed of 3 millimetres (0.12 in) thick steel with an aluminium alloy conning tower bolted to the top.

[4] Adding to the pilot's difficulties, the craft lacked compensating and trimming tanks, making staying at periscope depth a near impossibility.

[1] The submarine could be armed with either two TIIIc torpedoes with neutral buoyancy (achieved by limiting the number of batteries on board), mines, or a mixture of the two.

[1] Propulsion while submerged was provided by a 13 horsepower (9.7 kW) electric motor, supplied by three Type T13 T210 battery troughs.

[9] Operations generally lasted from one to two days with pilots either using a drug known as D-IX to stay awake on longer missions or caffeine-laced chocolate.

[12] An operation on the night 29/30 January resulted in damage to (much of it due to ice) or loss of most of the remaining Bibers.

[15] Vibrations from the U-boats’ engines caused the Bibers stern glands to leak allowing water to reach the machinery space and as a result the mission was abandoned.

[15] On 12 January 1945 Biber submarines were used in a night attack on the road bridge across the River Waal at Nijmegen.

The third Biber is privately owned and displayed outdoors at the entrance to Siegerpark in Amsterdam, it has been painted red and white and serves as an advertising sign.

Today, 5 of these are kept in original or restored condition at various museums: One at the Royal Norwegian Navy Museum, one at the Haakonsvern naval base, one at Kvalvik coastal fortress outside Kristiansund,[24] one at Tellevik coastal fortress[citation needed] outside Bergen and one at Søgne vicary outside Søgne.