In the afternoon, the position of the Tripoli group was established; a cruiser and two destroyers of Force B and two cruisers and two destroyers of Force K (Captain O'Conor, on the cruiser HMS Neptune) sortied at 18:00 to intercept. The force ran into a minefield 20 mi (17 nmi; 32 km) off Tripoli, in the early hours of 19 December. The minefield took the British by surprise as the water-depth was 600 ft (180 m), which they had thought was too deep for mines.