(NEWSER) – Puerto Rico's Mosquito Bay has long awed visitors with its magical glow, the result of plankton called dinoflagellates that "shimmer." This year, however, what Gizmodo dubs "one of the world's most spectacular natural sights" has gone dark. In January, the bay—located about 10 miles from the mainland on the island of Vieques—stopped glowing, and experts don't know why, the New York Times reports. And though the glow has recently returned somewhat, it's quite dim. That's bad news, of course, for the environment: Such bioluminescent sites are a rare phenomenon. It's also bad news for a tourist-dependent economy; hundreds of tourists normally gather daily at the bay.