Lake Rotorua is the second largest lake in the North Island of New Zealand by surface area, and covers 79.8km2.
The lake was formed from the crater of a large volcano in the Taupo Volcanic Zone. Its last major eruption was about 240,000 years ago. After the eruption, the magma chamber underneath the volcano collapsed. The circular depression left behind is the Rotorua Caldera, which is the site of the lake. Several other lakes of volcanic origin are located nearby to the east, around the base of the active volcano Mount Tarawera.
Owing to the geothermal activity around the lake (including still active geysers and hot mud pools), the lake has a high sulphur content. This gives the lake's waters an unusual yellowish-green hue.
Despite the large volume of water flowing through Lake Rotorua its shallow depth makes it very prone to discolouration, especially from sediment following windy weather. It is well used by fishermen, but less popular with watersports participants and swimmers.