The Blue Hole is an old quarry pit that fills with groundwater on the island of Big Pine Key, and it's now one of the most reliable spots in the Keys to see alligators. The quarry was dug in the early twentieth century for rock used in building the Overseas Railroad and later the Overseas Highway. When the work was done, it flooded and became the only significant body of freshwater in the lower Keys — which is precisely why the wildlife congregates here.
Two resident alligators have been living here for years and are frequently visible sunning on the banks or floating in the water. The surrounding area also attracts wading birds, turtles, and the occasional Key deer coming to drink. A short, flat trail runs around the quarry edge, making it an easy half-hour walk. The Blue Hole is located off Key Deer Boulevard within the National Key Deer Refuge.