One possibility that was feared at this point was that of breaking into the chamber too quickly, resulting in the water in the mine rushing upward and drowning the miners. An additional and possibly fatal danger was that of the miners being afflicted with decompression sickness, due to their breathing air which was at a higher pressure than the surface pressure, due to the pressure of the surrounding water. In preparation for these possibilities, an airlock was fashioned to go on top of the escape shaft, and on Thursday evening, 10 portable hyperbaric chambers arrived at the drilling site. Drilling continued until 1:38p.m. on July 27, when it was stopped to install the air lock and wait for the water to be pumped down to an elevation of mean sea level (“MSL”), approximately below the portal elevation.