Underground mining in perspective | All about Natural ...
Closed in 1941, some quarries were reopened in 1990 before acquisition by Polycor, Inc. A video on the Polycor website shows some of the underground workings. Figure 6 shows part of the site. RUSSIA. Underground mining of limestone was undertaken in Leningrad Region in the early 1990s but only as an adjunct to the mining of oil shale.