Treak Cliff Cavern
... that Treak Cliff Cavern is one of only two remaining active sources of the ornamental mineral Blue John?
... that <b id="mwBGs"><a href="./Treak_Cliff_Cavern" id="mwBGw" rel="mw:WikiLink" title="Treak Cliff Cavern">Treak Cliff Cavern</a></b> <i id="mwBG0">(pictured)</i> is one of only two remaining active sources of the ornamental mineral <a class="mw-redirect" href="./Derbyshire_Blue_John" id="mwBG4" rel="mw:WikiLink" title="Derbyshire Blue John">Blue John</a>?