In addition to four active volcanoes – Tongariro, Ruapehu, Ngāuruhoe and White Island – New Zealand has a very energetic thermal region centred on Rotorua. Already well known to Māori, these geothermal features became a popular tourist attraction in the late 19th century, despite their unusual sulphurous smell. Rotorua’s mud pools are the result of steam from deep within the earth rising to the surface beneath pools of rainwater. Clay is produced when nearby rocks are attacked by acidic gases, and this then mixes with rainwater to form mud. The rising steam continues to heat the mud to boiling point – the spectacular bubbling and explosive results resembling molten hot chocolate.