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seakingsoyuz t1_j5la4r5 wrote

This is total speculation, but anyway:

Underground warfare made a brief resurgence in WW1, because the front lines in Flanders were on top of soft chalk and the trench lines meant that the Western Front devolved into siege warfare. Counter-mining was also a serious concern, so the troops digging the tunnels would sometimes drill holes, soak the rock with vinegar to soften it further, then scrape away the top layer and repeat. This was a lot quieter than just hacking away with a pick.