At the beginning of June, dam operators opened the gates on the left spillway, sending , less than one-tenth of capacity, down the tunnel into the river below. After a few days, the entire dam suddenly began to shake violently. The spillway was closed down for inspections and workers discovered that the flow of water was causing cavitation – the explosive collapse of vacuum pockets in water moving at high speed – which was damaging the concrete lining and eroding the rock spillway tunnels from the upper ends of the diversion tunnels, which connect to the bottom of the reservoir. This was rapidly being destroyed by the cavitation and it was feared that a connection would be made to the bottom of Lake Powell, compromising the dam's foundation and causing the dam to fail.