571 Yama no Suigai [Floods in Coal Pits (Yama)]

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Floods in coal pits were sometimes caused by cave-ins of the bottom(s) of rivers or the sea.
Miners often said, “Furuto ni hogeta (My pickax went through the rock wall of an old cavity)’’ or “Ko-nai ni mizu ga abareta (Water flooded the pit)’’ in their dialect when they suffered from flooding. Coal pits were flooded many times, though these floods varied in scale. The main cause of these disasters was striking old cavities filled with water, which miners themselves had driven before. If there was not much water in the cavity struck by a miner, the miner could sometimes run away and evacuate to safer places without extracting his pickax. But in most cases, thin rock walls could not resist the water pressure, and pit workers did not have time to get away. Even the 2-meter-thick wall of a large old cavity filled with water could be holed by blasting, flooding the pit. The flood washed rocks and frames away, killing people in the pit with its fierce power.

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Article 395 of the Mining Safety Act required miners to do 5-meter-deep pilot boring when the distance to a cavity became about 50 meters. However, most floods in pits happened because miners did not know where the cavities were.
In the old days when cap lamps were not used in coal mines, the flooded slopes were plunged into darkness, making the flood damage worse.