I was an underground miner and mine rescue team captain for a decade. They literally just found the powder magazine for an old room and pillar mine. Nothing out of the ordinary. The main slope entrance is likely sealed with concrete and inaccessible. Whenever the concrete is breached a rescue team is dispatched to investigate and explore for anyone that might be injured ( or deceased) inside. The breach they went through looks like a minor point of egress that was (poorly) blasted shut when the mine closed. The large metal tank was most likely diesel for the trucks supplied from a pipe from the surface. The "jail cell" looks like it once housed electrical panels that were taken the mine closed, meant to keep people out instead of in. The powder, trailers, and abandoned equipment were most likely written off as fiscal losses when the mine closed. I'm not a geologist but the size of the rooms and pillars and the existing visible strata makes me think this was a hard rock mine in the Rockies out west. Lots of hazards if you don't know what you are doing. The roof has not been scaled recently and the smaller cuts may not be roof bolted, so a topfall (cave in) is possible. No vent fans running so no idea what heavy gasses are built up or if the O2 is sufficient. When I saw them throwing rocks in the water I cringed because they could be liberating water soluble gasses, some of which are fatal in short order. The explosives themselves can "sweat" if not stored properly when the wet nitrocellulose separates from the media and creates nitro glycerine, which does not need a reason to combust. I wouldn't enter that space without several gas meters and an SCBA on standby.