Start with a coastline.

Springs, ledges, and the deep artificial-reef program off the Panhandle and the west coast — including some of the world's premier cavern and cave diving.

Palm Beach to Miami: warm Gulf Stream drift dives, the wreck-heavy reefs off Pompano and Lauderdale, and goliath grouper aggregations.

Ancient amphora wrecks, WWII casualties off Malta, and the dramatic walls of the Medes Islands and Sardinia.

World-class wreck and reef diving — the SS Thistlegorm, the Brothers, and the coral gardens of Ras Mohammed.

The epicenter of marine biodiversity — Raja Ampat, Komodo, Sipadan, and the muck diving of the Lembeh Strait.

The continental United States' coral reef, running 120 miles from Key Largo to the Dry Tortugas — shallow reefs, the wreck trek, and the spiegel-class artificial reefs.

Atoll channels swept by current and packed with manta rays, whale sharks, and grey reef sharks across 26 ring-shaped atolls.

The shore-diving capital of the Caribbean — a fringing reef marine park you can dive from the beach, day or night, off Bonaire, Curaçao, and Aruba.

Puget Sound and the Salish Sea — giant Pacific octopus, wolf eels, and the cold, nutrient-rich water of the Emerald Sea.

High-voltage drift diving along the Mesoamerican Reef, plus the freshwater cenotes of the Riviera Maya — flooded caves of the Yucatán platform.

Giant kelp forests, the Channel Islands, Monterey's Breakwater, and cold green water full of life.

The largest living structure on the planet — ribbon reefs, the Cod Hole, and the SS Yongala, one of the best wreck dives anywhere.

Blue holes, shark dives, and dramatic wall diving across 700 islands — Nassau's wrecks, Bimini, and the Exuma chain.

Lava tubes, manta night dives off Kona, and the wrecks of the Mahi and the YO-257.

The greatest wreck-diving site on Earth — a ghost fleet of WWII Japanese ships sunk at anchor during Operation Hailstone, 1944.

Sheer walls dropping into the Cayman Trench, the USS Kittiwake, and the legendary Bloody Bay Wall off Little Cayman.

The southern Mesoamerican Reef: walls, wrecks like the Prince Albert, and some of the cheapest warm-water certification on the planet.

The RMS Rhone — the Caribbean's most famous shipwreck — plus the reefs and pinnacles of the Sir Francis Drake Channel.