Giant crystal cave in Mexico

In April 2000, brothers Juan and Pedro Sanchez were drilling a new tunnel when they made a truly spectacular discovery. While Naica miners are accustomed to finding crystals, Juan and Pedro were absolutely amazed by the cavern that they found. The brothers immediately informed the engineer in charge, Roberto Gonzalez. Ing. Gonzalez realized that they had discovered a natural treasure and quickly rerouted the tunnel. During this phase some damage was done as several miners tried to remove pieces of the mega-crystals, so the mining company soon installed an iron door to protect the find.

This page has more pics and a complete explanation...

The crystal cavern was discovered within the same limestone body that hosts
the silver-zinc-lead ore bodies exploited by the mine. The cavern was probably
dissolved by the same hydrothermal fluids that deposited the metals with the gypsum
being crystallized during the waning stages of mineralization. The crystals probably grew
relatively quickly to their immense size within a completely liquid-filled cavern.