Blue Hole National Key Deer Refuge

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.

The expanse of Big Pine Key’s Blue Hole

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.

Water framed by palms and trees with interpretive tags

Blue Hole sits in a tropical rockland hammock where rainfall collects in limestone pockets

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.

Key deer doe emerging from the forest at Blue Hole

Resources

Resources for exploring the area

South Florida Explorers Guide book cover

Hikers Guide to the Sunshine State book cover

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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.

Oily spots on the trunk of a mature poisonwood in the forest at Blue Hole

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.

A decidedly non-native iguana at the overlook

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.

Keys thatch palms along the trail

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.

The observation platform

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.

Florida cooter and green heron

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.

Near shore along Blue Hole

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.

Walking the worn path around Blue Hole

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.

Rocky ledge at the far end of the trail

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.

Path outlined by rocks through tropical forest

Native Plant Arboretum and Garden

Trail Map

Map of trails on Big Pine Key, National Key Deer Refuge — Mannillo Trail in yellow. Watson Trail in red. Blue Hole in blue.

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.