I run a couple of Street View based sites. Here are a few interesting and lesser known places which they've covered:

Wieliczka Salt Mine: http://www.instantstreetview.com/2n9vv9z3sayesz1uyzp0z2u

Battleship Island: http://www.instantstreetview.com/2bp3k8z5ve6kiz4bvzs4z2u

Lava cave in Japan: http://www.instantstreetview.com/2dcp2cz5x7dgmz15jzohz2u