A massive slab of rock is threatening to come crashing down at the base of an Arizona dam.

A three-person crew has been working to stabilize the sandstone that forms the walls surrounding the Glen Canyon dam near the Arizona-Utah border.

The slab weighs 500,000 pounds and recently began to crack due to erosion. The area below the slab includes a boat ramp, and water and power facilities for the dam.

The U.S. Bureau of Reclamation has cut off access to the boat ramp, putting rafting trips on hold until the rock can be secured.

The plan is to drill bolts that range from 6 feet to 8 feet long into the rock to secure it.