This one is a pretty popular spot in the Kootenays, I only learned of it recently just before embarking on a ~4300 km road trip to Saskatchewan. This quarry was such a big deal when it opened in 1909 that they named the area after it. It doesn't really matter which direction you approach Marblehead from, it's going to be awesome. 31 south along Kootenay Lake is incredible, 31A over to New Denver is incredible, 31 north to Trout Lake is incredible, can't go wrong in the West Kootenays.
The quarry ceased operations in the late 30s and has since existed as a popular party spot and photo op. Besides the scale of the place there isn't much to see inside, however, it is always fun to drive around inside of a mountain. Track it down next time you're out and about in the Koots.
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Stone from Marblehead lives on in one of Canada's most impressive structures, the Cardston Alberta Temple: