Rotorua sits on top of the most active segment of the Taupo Volcanic Zone - an active thermal belt in the South Pacific.  Wherever the earth's crust allows, you'll see thermal activity.  That includes on the golf course, in the center of town, and in people's backyards!  New ones are always cropping up, and in many places we saw that the city ropes them off to keep people from injuring themselves.