Earthquake

The explorers exterminating the Scorched Earth Rats had made their method effective enough to split up into smaller groups and spread out through the wasteland. Instead of gathering the rats densely, like they had done at first, they poured water and a light amount of explosives into the tunnels in a much larger area.

With enough experimentation, they had managed to create a liquid explosive that slowly began drying up in contact with air, leaving behind a highly flammable dusty substance that wouldn't mix with other materials after drying.

The explorers first poured down that mixture, and after it dried, leaving a trail of fuses, they poured various other explosives that were efficient at collapsing the tunnels. Since the second round of explosives tended to be thicker and flowed slower, they were flushed down with water.