Pukaki Lagoon has a 600 m wide explosion crater, with a surrounding tuff ring. After an eruption about 65,000 yrs ago the crater filled with freshwater and became a lake. It was breached by the sea as sea-levels rose after the end of the last ice age about 8,000 yrs ago and became a tidal lagoon. This was dammed and drained and used as a speedway from 1929 into the 1930s (Henning's Speedway). It is now farmland.