When I first heard about the San Jose mine at Copiapo, Chile, I assumed it was like other deep metal mines I've dropped in on over the years, in Nevada, Montana and Minnesota. In traditional deep shaft mines, access is usually gained by riding a cage down a vertical shaft. In mountainous regions, access is sometimes through adits (horizontal tunnels) excavated into the side of a hill.
Using a technique called stoping, some shaft-mine operators have chased rich veins from deep deposits all the way to the surface, leaving a very scary exposed chasm like the one in the old movie Taras Bulba. One abandoned gold mine I saw in New Mexico had blasted out a crevice, at least eight feet wide that ran for hundreds of feet along the tree-lined valley. Anyone who stumbled across that one in the dark was heading for the bottom.
Based on the diagrams and news stories being published, though, the 121-year-old San Jose mine was none of these. Call it a corkscrew mine, because access to the deep elevations (that is, before the collapse) was by driving down a sloping, spiral tunnel. This pathway stays mostly within the giant ore body and gives access to side tunnels. Here's a closer view of the 3D layout from the Daily Mail.
Passing near the pathway route at irregular intervals are vertical ventilation shafts. From the diagrams I saw some elevations of the mine have three ventilation shafts in parallel; at other elevations there is just one; and at some elevations there are no ventilator shafts at all, so air can only get through that level of the mine by following the pathway. Two days after the initial collapse of part of the spiral pathway on August 5 (which may have blocked one of the ventilator shafts at the same time) rescuers tried to slip past the collapse zone by using the only other navigable ventilation shaft at that level, but their efforts triggered that shaft to fall in as well. Confusing! And frustrating!
That left no other expeditious way for rescuers to reach the miners besides setting up their big drills away from the hollowed-out area and its spiral pathway. Using some pretty amazing navigation they angled the masts slightly so as to intersect a portion of the mine that's 2,000 feet deep and that offers access to the miners' refuge. The 26-inch bit from the Schramm T-130XD Rotadrill rig that broke through yesterday opened an escape route through the ceiling of their workshop. Given the bit's hammering and a stream of gravel that had been coming down the hole for days, there was no doubt among Los 33 that help was on the way.
While it may seem that rescuers could have saved themselves some time by taking a shortcut and just drilling through the maze of pathways and old ventilation shafts, the debris piled up in old excavations is pretty sure to damage or jam the drill bit. In fact an encounter with such debris 880 feet down in one old section of the mine stopped the Schramm operation for several days in early September.