This is inside the Crater in Midway, Utah. It's a warm spring (90-95˚F and mineral laden) that's very deep (65 feet) and used for scuba diving training. Originally the level of the pool was much higher, spilling out of an orifice at the top of a 50ft tufa mound, but years ago the owners dug a horizontal tunnel through to the interior (and diverted the water) to create the space for diving. The interior is lit by natural light from the open top, and by lights, providing interesting effects. In this photo I tried to balance the scuba gear against the mysterious, dark right side of the photo, with the gradient of shadow provided by the lighting connecting the two.
John Christensen