Inside one of the two summit craters that make up Lassen Peak in Lassen Volcanic National Park, California.  The area was still smoldering during the May 1915 eruption & white sulphur deposits are visible on the south rim of the crater.  This shallow crater is easily visible from the main summit area where visitors ascend but the small lake itself at the crater bottom is well concealed, something I wasn't expecting to find while roaming around the crater rim.