Asd took the first step down and didn't hesitate as he led everyone in a single file, the first caves were within 50 meters of going down, there was a change of temperature the very moment they stepped onto the ramps and the beams that supported them. while the exact nature of the fog wasn't known, the fact that it was attracted to heat of any significant source would and could explain the drop in temperature after they went down the basin. Either way Asd had to keep his focus sharp as they marched down, he had to keep his eye out for the entrance to the tunnel that would lead to the right path.