Each plane carried one of Barnes Wallis’s ‘bouncing bombs’ – a kind of gigantic depth charge that would skip across the water to hit the dam wall and sink 9 m (30 ft) before exploding. It had to be dropped precisely 18 m (60 ft) above the water and 388 m (425 yds) from the dam. Each Lancaster was fitted with angled lights on its nose and rear fuselage, whose beams would meet on the water when the plane was at the correct height . Bomb-aimers used various makeshift techniques to judge the distance.