2 o'clock
The outer of the double doors was opened and the four men stepped in , entering a tiny ante-chamber . They closed the outer door , which was absolutely air-tight , opened the inner one , and passed into the chamber built against the coal face , made of solid cement except for a circle of coal a yard in diameter . A minute or two later , could be heard , faintly , the high screech of some rapid-cutting machine . When Otto came back on his next shift , at 2 o'clock on Friday morning , the sand hogs were still working . Curiosity overcame the old miner 's desire not to seem ignorant . `` Just what is that , sir ? '' he asked the Bureau official , who was still on watch .