On tropical islands with karst landscapes, a blue hole is a type of sinkhole where a lens of fresh water floats atop a layer of salt water seeping in from the sea.
The fresh water comes only from rainfall, so the blue hole acts like a natural cistern.

Think of Big Pine Key as a giant sponge with lots of small surface holes. Fresh water soaks into the limestone karst 22 feet deep, with salt water lying below it.
Early settlers dug shallow wells to trap this natural resource, but the population has long outstripped the availability of fresh water on Big Pine Key.

While Big Pine Key has the proper geology to create a natural blue hole, this particular Blue Hole is man-made. It formed within a quarry where rock was mined for construction of the Overseas Highway.
By exposing the fresh water lens in the karst, Blue Hole became the primary source of fresh water for the wildlife of Big Pine Key.

Resources
Resources for exploring the area
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Overview
Location: Big Pine Key
Length: 0.3 mile round-trip
Trailhead: 24.7060, -81.3800
Address: Key Deer Blvd
Fees: Free
Restroom: None
Land manager: National Key Deer Refuge
Phone: 305-872-0774
Open daily from a half hour before sunrise to a half hour after sunset. Repellent is a must.
Bicycles are not permitted on the trail, and pets are not advised so as not to scare away wildlife. Do not swim in Blue Hole. There are large alligators living in it.
Report wildlife emergencies such as injured or dead Key deer or aggressive animals to Florida FWC immediately at 888-404-3922; press Monroe County extension.
Stay within the marked path to avoid brushing into poisonwood.
Poisonwood is found along this trail, and is much more irritating than poison ivy. Its fruit is an important food source for endangered white-crowned pigeons but deadly to humans.

Directions
From US 1, follow Key Deer Boulevard past the shopping center. Pay attention to the speed limits as they are slow for a reason: you may see Key deer cross the road along this stretch, especially at dusk and dawn. The turnoff for the parking area is on the left after 3.1 miles.
Hike
Two trails lead from the trailhead kiosk. The one to the right is a short spur to an overlook that gives you a quick glance over Blue Hole.
It may be obscured by vegetation, but it’s also the first place you might see wildlife.

Return to the kiosk and take the trail to the left. When it forks, keep to the right.
The footpath winds through an area planted with native trees such as pigeon plum, gumbo limbo, and silver buttonwood.

You quickly reach the observation platform overlooking Blue Hole, which is as far as many visitors get.
The panorama takes in the entire water-filled quarry, only shallow enough for cattails along its near edge.

Look straight down. The water is clear, and hosts a constant parade of aquatic life. Large Florida softshell turtles drift through the shallows.
Bluegill, bass, and bream dart between the coontail. A massive giant oscar noses up to the platform piers.

Watch for the bobbing heads of Florida cooters and the Florida mud turtle as they scoot across the limestone bottom.
If an alligator surfaces, don’t be surprised. They like the shade that this deck provides.

Leaving the platform, follow the worn path to the right. It makes an arc around Blue Hole. It’s in this area that we’ve most commonly seen Key deer grazing.
At Blue Hole, deer have access to copious amounts of fresh water, no matter how dry the adjacent pine rocklands may get. That’s not true in the rest of the Lower Keys.

At a Y junction where a paved road is visible on the left, keep right. There are several open spots with clear views across the pond.
The trail ends at an “Area Closed” sign with a fence, blocking off the rest of the quarry road to enable deer to reach Blue Hole without human disturbance.

Return the way you came, but take the natural path to the right. This loops through the native plant garden and arboretum.
The names of the plants are mapped out on the kiosk by the trailhead. The full round-trip and loop is slightly more than a quarter mile.

Trail Map

Explore More!
Learn more about National Key Deer Refuge

National Key Deer Refuge
Established in 1957 to prevent the extinction of the diminutive Key deer, National Key Deer Refuge spans 84,351 acres across 25 islands in the Lower Keys.
Slideshow
See our photos from Blue Hole
Nearby Adventures
More worth exploring while you’re in this area.

Fredrick C. Mannillo Jr. Nature Trail
Key deer browse the understory of a pine rockland explored on this short accessible interpretive trail within National Key Deer Refuge.

Jack C. Watson Wildlife Trail
Rare pine rockland and rockland tropical hammock are key habitats for Key deer along this immersive interpretive loop on Big Pine Key.

Overseas Heritage Trail, Lower Keys
38.0 miles. Between Key West and Bahia Honda Key, this segment of the Florida Keys Overseas Heritage Trail crosses many historic railroad bridges in view of mangrove-lined shores.

Bahia Honda State Park
Long known for its natural beaches, Bahia Honda State Park continues to heal after the ravages of Hurricane Irma stripped much of its tropical vegetation