The Nazis knew of Oscarborg and its antiquated guns but had no knowledge of its hidden torpedo battery. A natural cave had been enlarged and enhanced with three torpedo chambers loaded via underwater cage elevators. The Blücher unwittingly passed with 500 m (550 yards) of the torpedo battery. Colonel Andreas took aim and sent two torpedoes racing towards their target, hidden underwater at a depth of 3 m (9.8 ft).