on foot. A third explosion occurred at 3:39 pm on 26 November 2010; it appeared to be smaller than the first two. A fourth significant explosion ignited the coal within the mine; the subsequent fire was visible above the ventilation shaft; the steel structure above the shaft was damaged and neighbouring scrub set alight. The fire appeared to be located near the bottom of the shaft, burning either loose coal or the seam itself, and considerably complicated efforts to stabilise the mine and made recovery of "intact" bodies unlikely. Gas monitoring The initial explosion damaged the mine's gas drainage line, causing methane gas to begin accumulating in the mine immediately. As there may have been a potential ignition source, it was too dangerous for rescuers to enter the mine. It was originally predicted to take several days before the mine was safe enough for rescuers to enter, as the gases inside were feared to be explosive. Initial testing at the mine ventilation shaft was hindered by heavy clouds, preventing helicopter access, and staff were going to have to walk in over rough terrain, as the shaft does not have road access. Seismic equipment was attached to tubes at the tunnel mouth to detect movement in the mine. With tests still not giving clearance for rescuers to enter the mine, an attempt was made to enter the mine using a bomb disposal robot provided by the New Zealand Defence Force (NZDF). The robot failed only into the mine due to water ingress. Sources noted that while the robot was capable of operating in rain, it had "effectively [been] hit by a waterfall", short-circuiting it. A second NZDF bomb disposal robot was placed on stand-by to enter. This robot had been fitted with extra batteries and other equipment to try to avoid the problems which hit the first robot. It was later deployed, and the first robot was later restarted. The robots entered the mine on 23 and 24 November 2010, while a third, from Australia, was en route to the site. The use of three robots was unprecedented in mine rescue. The use of United States mining rescue/exploration robots was also being considered though the second explosion later that day effectively ended the robot efforts. Early on 24 November 2010, it was reported that a drill started from above the horizontal mine had reached to the mine chamber, releasing hot gas.