Info from wikipedia:
The Oaks Colliery, which was one of the largest coal mines working the Barnsley area in South Yorkshire Coalfield, mined a seam that was notorious for firedamp. Almost 20 years before, on 5 March 1847, The Oaks colliery suffered its first disaster when a blast killed 73 men and boys. As mine management was aware of firedamp, there were strict rules about the use of safety lamps. A ventilation system was also used to carry any gas that emerged from the seam out of the mine. However the coal in this seam was known to contain methane making it a very dangerous work.
On Wednesday 12 December 1866, 340 men and boys were working the day shift. With less than an hour of the shift remaining, a huge explosion ripped through the workings. The force of the blast blew the cage up No. 1 shaft into the headgear, breaking the coupling. The cage was recovered and replaced to enable a party of “pit deputies” (foreman) to descend the pit to see the devastation. At the bottom of the shaft, they found a number of badly burned men who were sent up to the surface. The dead were taken to their homes and the survivors were given medical attention. By midnight, the exhausted rescuers withdrew to continue their work the next day.
The next morning, 27 rescuers went down the pit with Mr Minto, the underviewer, and mining engineer Parkin Jeffcock to inspect the conditions under which they were working. But as Jeffcock finished inspecting the upcast shaft, another huge explosion occurred killing all the rescuers. The blast was powerful enough to rush up all three shafts at the colliery. A third explosion took place a few hours later, again affecting all three shafts.
In total the explosions killed 361 miners and 27 rescuers. Among the many dead were the pit ponies and their boy handlers, who hauled wagon loads of coal from the workings to the mine shaft. They had all been killed in the first explosion.