MARATHON, Florida Keys — While there’s great diving throughout the Keys island chain, Marathon — known as the Heart of the Keys — offers an intriguing mix of shallow-water reefs and shipwrecks sure to please the most discriminating diver.

Crystalline waters off Marathon are flush with extensive spur-and-groove coral formations and well-developed patch reefs that demand hours of exploration. High-quality shallow reefs are heartily populated with tropical reef fish — many say they’re even more plentiful than in Caribbean waters — as well as invertebrates and soft and hard corals. Here are some locals’ underwater favorites.

Coffin’s Patch encompasses a series of six formations running northeast and southwest in less than 30 feet of water. Snorkelers and divers alike can explore elkhorn corals, pillar corals and an abundance of organisms amidst the tongue-and-groove structures in this area, designated a specially protected area, or SPA, by the National Oceanic and Atmospheric Administration.

Sharing the same name as the 150-year-old lighthouse that adorns it, Sombrero Reef also is an SPA and is Marathon’s most popular underwater attraction. Its coral canyons, archways and coral heads are splayed out like fingers among sand spits, providing refuge for burrowing stingrays, reef fish like sergeant majors, blue tang, grunts, snapper, silver-streaked barracuda, resident moray eels and the occasional reef drifters, sea turtles.

Horseshoe Reef, named for the shape that coral heads are situated in, is a haven for nurse sharks to gather and feed. Horseshoe is where new divers will find it easy to navigate, its sandy middle easy for practicing skills, training as well as critter-sighting during night dives. Octopus, shrimp, crabs and the occasional conch wander after twilight.

Both modern and historical shipwrecks complete the Middle Keys’ dive appeal. Shallow-water wrecks such as the Adelaide Baker and Flagler’s Barge are replete with porkfish (a member of the grunt family) and wrasse, but most notable of Marathon’s scattered wreck sites is the Thunderbolt, a 188-foot cable layer that was sunk intentionally as a dive attraction in March 1986. The vessel lies in approximately 120 feet of water 6.5 miles south of Duck Key Channel in the Florida Keys National Marine Sanctuary. Today it is coated with colorful sponges, corals and hydroids, providing refuge and sustenance to large angelfish, jacks, cobia, tarpon and a variety of deep-water pelagic creatures.

Originally named Randolph, the Thunderbolt served as a research vessel to explore the electrical energy of lightning strikes — hence its name.

An entirely navigable wreck, the Thunderbolt is accessed by following the lines of two submerged mooring balls positioned at the bow and stern. The top of the observation deck is 75 feet beneath the surface. The Thunderbolt sits upright with large angelfish patrolling its decks and notable giant cable spool at the bow; barracuda stand watch in the wheelhouse, an easy and intriguing swim-through. The aft end of the wreck has been cut away to expose the engine room and the interior of the hull.

Perhaps the most popular residents are the three goliath grouper that hang around the engine room — one nearly 800 pounds. Schools of amberjack, plentiful hogfish, black grouper and the occasional reef shark reside near the rudder and propellers, left on the ship to complement the stern section of the hull to appeal to divers.

A variety of sites in the waters off Marathon offer divers of every level a memorable recreational experience. Shallow reefs offer the benefit of longer bottom times and vibrant colors, while the shipwrecks provide a glimpse into the maritime history of the coral island chain.

To arrange a dive, contact one of the Middle Keys’ dive operators. All are staffed with working professionals who dive every day and can provide a unique introduction to the region’s underwater world.