Just a 5 minute walk from Rotorua city center is Sulphur Bay at the southeast end of Lake Rotorua.
A strangely eerie place. The water is acidic, sulphurous and mineralised and small fumaroles can be seen, as can acid etched bits of wood (quite beautiful). Of course being close to town there is the inevitable examples of random rubbish like the kids sandal below.
The area has history and the link will take you to more info