Hi,

All mines deloyed by UC Class U-Boats were Contact mines. They consisted of a metal casing approx containing about 350lbs of TNT or amatol, and an amount of air for bouyancy. In the middle of the mine was a battery attached to a detonator on the mine surface. The Horns, of which there were 4-7 depending on model were brass alloy tubes which contained a sealed "test tube" sized glass vial. This contained either potassium chromate (an electrolyte)or Chromic Acid.
These horns were known as Hertz Horns after their 19th Century German inventor.

The battery within the mine was dry, which meant that there was no electrical current generated without the electrolyte. When a ship passing crushed one of the horns, the glass vial broke which sent fluid to the battery making it live. This in turn activated the detonator, which in effect was a delayed reaction, and the explosion was usually amidships to devastating effect.

The mines were contained in carriages which served as a sinker and anchor. The mine was attached by a cable to the sinker through a reel on the sinker. When discharged, the mine and sinker wold both go to the seabed. A plug of salt would gradually dissolve and the sinker would release its cable to the prescribed depth.

They were usually laid in groups of four by a u-boat, generally at harbour entrances or restriced channels, and the maximum laying depth was about 120ft.
The whole point ot these mines was that they would catch an unwary ship, so the intention was to leave the mine itself under the surface, not bobbing about for the sweepers to find.