(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.

One theory is that winds in the area recently shifted northward, which could have propelled the dinoflagellates into the Caribbean Sea, says a marine biologist. The winds also increase the bay's turbidity, impeding bioluminescence. The blackout has heightened tension between locals and tourist companies that visit the area. Tourists still have permission to visit, but locals now have to pay the companies to see the spot, too, says a fisherman. As for whether the bay's current weak glow will strengthen again, history may not be much of a guide: Some of the planet's bioluminescent bays have resumed their glow after darkening; others have stayed completely dark. (In the meantime, check out four other neat places to swim, including one where you'll be alongside "monster jellyfish.")

