WHEN YOU SLIP INTO THE OCEAN AND SWIM NEXT TO A whale shark, it’s the underwater equivalent of going on safari, then getting out of the Land Rover and running with elephants. Shark spotters in search of this ultimate in-water encounter travel far and wide for a chance to dwarf themselves next to these massive creatures. In spite of their status as the world’s largest fish -they grow as long as 50 feet-the animals are shy and spend much of their lives hidden from view. But in spring and summer the big-mouthed behemoths arrive en masse to feed at a few choice locations in the western Caribbean and in the Gulf of Mexico.

Flower Garden Banks National Marine Sanctuary, Texas

An oasis of life in the center of an oceanic desert, the Flower Garden Banks National Marine Sanctuary lies about 110 miles off the coast of Freeport, Texas-so no passport needed. The area attracts a slew of creatures and boasts such unique animal life that in 1992 it became one of 14 federally protected marine areas.

WHEN TO GO: Whale shark sightings at the Flower Gardens aren’t as common as at locations farther south, but a good time to try is in August and September. WHO TO GO WITH: Because the site lies so far offshore, the best way to experience it is on a live-aboard boat. Fling Charters offers two-and three-day trips (four and five days during coral spawning) aboard its 100-foot dive boat, the MV Fling (from $805; flingcharters.com). Trips include two to five dives a day, meals, and snacks.

Isla Holbox, Mexico

Each summer hundreds of whale sharks congregate off this island to feed on plankton blooms. Isla Holbox sits just a few hours’ drive northwest of Cancun, yet the atmosphere in this fishing community is a world away from its developed neighbor. There are no cars on the island; the only motorized transportation is a golf cart.

WHEN TO GO: May to September WHO TO GO WITH: Holbox Tours and Travel offers money back guarantees on snorkeling encounters from June through August (from $95; holboxwhalesharktours.com), making this destination your surest bet. WHERE TO STAY: Casa Iguana is a private beachside retreat surrounded by palm groves (from $55; casa-iguana.net). WHERE TO EAT AND DRINK: Edelyn’s, a hopping pizzeria, is located in front of the main square. Fill up on Chef Miguel’s signature lobster pizza, and when you’re finished, walk to local hangout Habana Nights for a nightcap.

Utila, Bay Islands, Honduras

Along Utila Town’s dusty roads, guesthouses and dive shops tout the seasonal appearance of the island’s biggest attractions: whale sharks. These giants feed on plankton blooms by following schools of tuna. Watch for fish “boiling” at the water’s surface: There’s a good chance you’ll find a whale shark circling underneath.

WHEN TO GO: February to June; August to October, WHERE TO STAY: Get an all-inclusive room and dive package (a great deal) at Utila Lodge, and hop on the boat straight from your waterfront bungalow along the private dock (from $209 per night; utilalodge.com), WHERE TO DRINK: At the Tranquila Bar, an open-air tavern suspended over the water, island expats and backpackers come together to down Flor de Cana rum and deconstruct the day’s dives.

Gladden Spit Marine Reserve, Placencia, Belize

Belize’s Mesoamerican barrier reef, the largest in the Western Hemisphere, lies just off Placencia’s shore and offers stunning natural scenery: The Gladden Spit Marine Reserve, a vital section of the reef, hosts the annual spawning of more than 25 species of fish. During the full-moon spawning, whale sharks gorge themselves on clouds of eggs and sperm. Sign up early: Tours fill up months in advance.

WHEN TO GO: March to June WHERE TO STAY AND DIVE: The Inn at Robert’s Grove boasts a dive center with a fleet of fast boats that dock on 22 acres of beachfront property five miles from Placencia Village (from $240 per night; robertsgrove.com). TOPSIDE ADVENTURES: Placencia makes a perfect base for jungle trekking up the Monkey River, exploring the Cockscomb Basin Wildlife Sanctuary (the world’s first jaguar preserve), and hiking the Mayan ruins of Lubaantun.

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