New Zealand is located exactly on a junction of Pacific and Australian tectonic plates.
Therefore, seismic processes are very common. Volcanic activity, earthquakes, tsunamis and landslides are no wonder to locals.
In places like Rotorua, crust of the Earth is thin, allowing plenty of thermal areas to occur.
They say it's good for skin to apply some of substance taken out of the pool with boiling mud, or take a bath in a steaming hot water river.
Sounds like heaven.
But when you get a breath of the ever present smell of sulphur in the air, it smells more like hell.