The "mountain" in the background is actually the rim of the volcano, which is partially blocked by clouds. The flat area in the background is the sand-like lava bed of the volcano. The structure there is a Buddhist temple - a mostly flat area a bit smaller than a football field, fenced around and empty.
The slope on the left and this vantage point are both volcanic peaks rising out of the lava bed. This stairway and that temple are the only man-made structures in the crater.
Mt. Bromo, Java. April 1996.