Sterling Hill was the last working underground mine in New Jersey. It closed in 1986 and reopened as a museum, and what it is famous for is what happens when you turn the lights off.
The ore bodies sit in the Franklin Marble, within the Reading Prong massif. That rock started as limestone in a Precambrian oceanic rift trough and was then heavily metamorphosed during the Grenville orogeny, and the result is a mineral assemblage of extraordinary variety. Sterling Hill and the neighbouring Franklin Mine are known worldwide for that variety, and above all for the fluorescent minerals. The museum cut a passage specifically to show them: the Rainbow Tunnel, driven in 1990 using 49 blasts at a cost of $2 a foot, where short wave ultraviolet lights are switched on and the entire tunnel glows.
Mining here goes back to the 1630s, when the deposit was mistakenly taken for copper. George III granted the property to William Alexander, Lord Stirling, who sold it to Robert Ogden in 1765. It passed through several hands before the various mines were consolidated into the New Jersey Zinc Company in 1897.
The end was not geological. The mine closed in 1986 over a tax dispute with the town, which foreclosed for back taxes in 1989 and auctioned the property. Richard and Robert Hauck bought it for $750,000 and opened it as a museum in August 1990. It went onto the National Register of Historic Places on 3 September 1991 for its industrial significance.
The site holds rather more than the mine: the Thomas S. Warren Museum of Fluorescence, an astronomical observatory, and a large collection of mining equipment. Tours go into the workings on foot.
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