The 1995-1996 eruptions of Ruapehu in the North Island of New Zealand left a 7-metre high dam of tephra, consisting of volcanic ash and rock, around the rim of the crater lake. It was realised that as the lake refilled and rose above the level of its normal outlet, the tephra dam would eventually collapse, causing a large lahar. Such a lahar resulted in the 1953 Tangiwai disaster when 151 people lost their lives as the lahar swept a railway bridge away, causing a passenger train to plunge into the Whangaehu River. In 2000 the government decided to plan, design and implement ERLAWS - a complex system of sensors and preventative mechanisms to warn of an impending lahar.