What is the biggest key in the Florida Keys?

What was the man-made hole on the Spanish Harbor Keys in the Florida Keys used for?

• In the Keys just past the Seven Mile Bridge are 2 small islands called the Spanish Harbor Keys. Off one is what seems to be a quarry of some type that is now popular for fishing, snorkeling and diving. I haven't been able to find out anything about this online. Does anyone know how old it is? What it was used for? How deep it is? I am very curious! Thank you!

• Answer:

  More or less known as the horseshoe, the quarry was one of many pits dug for fill by the railroad during construction of the Overseas Railroad between 1905 and 1912, financed by Henry Flagler. It's about 35 feet deep. You may find this 10 page article of the construction of the Overseas Railroad interesting. With old photos: http://keyshistory.org/caserailroad.html Some of the original quarry machinery is on display at Windley Key Fossil Reef Geological State Park in Islamorada.