One mile northwest of U.S. 1 on Lower Sugarloaf Key (mile marker 17) is where you'll find this kooky old structure. It's the Sugarloaf Key Bat Tower, built in 1929 by fish camp owner Richter Clyde Perky. Perky's idea was to raise bats in hopes they'd help control the problematic mosquito problem there. He purchased the plans from a Texas bat enthusiast named Charles Campbell, but unfortunately the tower didn't work - as soon as Perky installed the live bats he had shipped in, they flew away and never returned.
According to Wikipedia:
There are three Campbell bat towers still standing (out of an original fourteen world-wide) in the United States: the Perky Tower; one in Comfort, Texas; and one at the Shangri-La Gardens in Orange, Texas. At least one of the Texas towers has been internally reconstructed so that bats currently roost in it. The ruins of a fourth Campbell tower, in Temple Terrace, Florida, burned in 1979 and now consists of the concrete base/legs. Temple Terrace is in the process of rebuilding their 1924 tower.
Perky intended to found a city named after himself, but gave up after his inability to cope with the swarms of mosquitos. It's a shame his bat population program didn't work out - bats love pineapple and back then, the predominant pineapple was the "sugarloaf" variety from which the island gets its name. The sugarloaf pineapples are soft and easily broken into, and the fruit inside (white, not yellow) is said to be so tender you can eat them with a spoon, like watermelon. Sadly, the sugarloaf was almost entirely phased out of modern agriculture in favor of today's firmer-fleshed types that are more durable for travel. (It's making a comeback though, and can be easily ordered from African and Hawaiian sources!)