Mountain “bumps” and a second mine collapse that killed three rescue workers on Thursday, has prompted officials in Utah to suspend the underground search for six miners missing since an August 6 cave-in.

Six others were injured in Thursday’s collapse as they were tunneling through rubble to reach the missing miners. Officials on the scene at the Crandall Canyon Mine in Huntington say no more digging will take place until the safety of the crew can be guaranteed.

Before Thursday’s second collapse, cameras had been lowered into the mine, indicating spaces with enough air and water to sustain life. Microphones also picked up unknown noises.

A mountain bump, also called a bounce, is a seismic event common in coal mining.