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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.

Wooden platform for viewing thermal activities, near Rotorua, New Zealand

They say it's good for skin to apply some of substance taken out of the pool with boilind mud, or take a bath in a steaming hot water river.
Sounds like heaven.

Boiling mud in fiery explosion, near Rotorua, New Zealand

But when you get a breath of the ever present smell of sulphur in the air, it smells more like hell.

Smaller mud eruptions usually have nicer forms, near Rotorua, New Zealand

Believers are saying that this hot mud is very healthy to skin, near Rotorua, New Zealand