Mount Ruapehu, at the southern end of the Taupo Volcanic Zone, is one of the most active volcanoes. It began erupting at least 250,000 years ago. In recorded history, major eruptions have been about 50 years apart, in 1895, 1945, and 1995-1996. Minor eruptions are frequent, with at least 60 since 1945. Some of the minor eruptions in the 1970s generated small ash falls and lahars that damaged ski fields. Between major eruptions, a warm acidic crater lake forms, fed by melting snow. Major eruptions may completely expel the lake water. Where a major eruption has deposited a tephra dam across the lake's outlet, the dam may collapse after the lake has refilled and risen above the level of its normal outlet, the outrush of water causing a large lahar. The most notable lahar caused the Tangiwai disaster on December 24, 1953, when 151 people aboard a Wellington to Auckland express train were killed after the lahar destroyed the Tangiwai rail bridge just moments before the train was due. In 2000, the ERLAWS system was installed on the mountain to detect such a collapse and alert the relevant authorities.