The gray pile on the left is the tailings from a Marble Canyon dam site drift.
A exploratory tunnel (drift) drilled to determine the feasibility of a dam in Marble Canyon. Happily, unlike Glen Canyon, this never happened... The drift goes back a long, long way and forks in several directions not all that far back. There are still some tools from the effort lying around the site.
The first hub you come to in the drift. This area is permanently dark, so if you don't know which way you came in, you'd be in trouble. No light at the end of the tunnel. You start to have an inch or so of standing water here on on the floor of the tunnel. In the center of the floor is something that looks a little like a silt covered wooden hatch. I wouldn't step on it...
