The entire city of Rotorua is located on an active volcanic field. (Lake Rotorua itself is a giant caldera.) In some places the molten magma is only 50m below the surface. Steam vents and mud pools and boiling lakes can be seen throughout the city, although most of them have been cordoned off into parks as tourist attractions. Here we see a mud bubble exploding from the pressure (it took a lot of patience and a lot of photos to get the timing right).