West Virginia ? Two days after the worst U.S. mining disaster in a generation, rescue crews were called to the Upper Big Branch coal mine late Wednesday to prepare to descend underground and search for possible survivors of the explosion that killed 25.

Chances were slim that any of the four still-missing miners survived, because of poisonous gases in the underground tunnels. But a federal safety official said efforts to drill holes and ventilate the mine had resulted in lower levels of lethal carbon monoxide and highly explosive hydrogen and methane at the top of the holes.

Officials planned to test levels at the bottom of the holes to determine if six teams of five rescuers each could safely enter and look for the missing.

The hope: that survivors made it to one of the shaft’s rescue chambers, which are stocked with food, water and enough oxygen to last four days. As of late Wednesday, there had been no signs of life deep underground.