The Yucatán Peninsula is truly a freak of nature -- a flat, nearly 134,400-sq.-km (51,900-sq.-mile) slab of limestone with almost 1,600km (1,000 miles) of shoreline that is virtually devoid of surface water. The peninsula's geology, found nowhere else on Earth, was shaped by the same meteor thought to have extinguished the dinosaurs 65 million years ago. The impact fractured the brittle limestone into an immense network of fissures that drain all rainwater away from the surface. You'll see no bridges, no rivers, lakes, or streams in the northern and central Yucatán, but fresh rainwater courses through a vast underground river system stretching for hundreds of miles.

Breaches in the ceiling of this subterranean basin have created an estimated 3,000 cenotes -- sinkholes that reveal the underground to the world above. The Maya called them dzonots, or sacred wells, and regarded them as gateways to the underworld. Precious stones, ceramics, and bones unearthed in cenotes suggest that they were ancient ceremonial sites.

Quiet, dark, and cool, cenotes offer respite from the bright, often steamy glare above. Some are underground, with only a small breach in a roof perforated by thirsty tree roots. Others open to the surface like a lake. Most tourists get their introduction to this subterranean world at Chichén Itzá, with its Grand Cenote, and Hidden Worlds Cenotes Park, with underground caverns and waterways north of Tulum. But thousands more lie at the ends of narrow dirt roads throughout the peninsula, the greatest concentration being inland from the Playa del Carmen-Tulum corridor. The largest is Cenote Azul, at the end of Laguna de Bacalar in southern Quintana Roo.

The thin layer of soil coating the Yucatán's limestone shelf supports a nearly uniform terrain of dense, scrubby jungle full of wild ginger and orchids, jaguars, monkeys, and tropical birds. The only elevation is in the Puuc Hills. Rising south of Chichén Itzá and extending into northwestern Campeche, they peak at less than 300m (984 ft.).

The geography changes abruptly at the peninsula's isthmus. Hot, marshy Tabasco is a low-lying state bordering the Gulf of Mexico. With about 30% of Mexico's surface freshwater, Tabasco is said to be more water than land. Parts of the state are cloaked in thick rainforest, which extends through Chiapas. The capital, Villahermosa, lies in a shallow basin about an hour from the coast at the confluence of two rivers. Small lakes break up the landscape, especially in the modern parts of the city.

Chiapas, a much larger state of wildly varying elevations, extends from Tabasco all the way to the Pacific on one side and Guatemala on the other. It is washed by abundant lagoons, waterfalls, and rivers, including the Usumancinta, which forms the border with Guatemala. The high central plateau of a dramatic, pine-covered mountain range is the uncommonly chilly domain of San Cristóbal de las Casas, a small and ancient colonial city surrounded by Maya villages. Nearby Sumidero Canyon is a winding river gorge with some walls reaching 1,000m (3,281 ft.) into the sky. Palenque, one of Mexico's most exquisite ruins, sits where the northern highlands slide down to the Gulf coastal plain.

Southern Mexico is blessed with an astounding diversity of wildlife. North America's only two flamingo breeding grounds flank the northern Yucatán's Gulf Coast, and whale sharks convene off the peninsula's northeastern tip. Jaguars, howler monkeys, crocodiles, sea turtles, and hundreds of bird species populate the Sian Ka'an Biosphere Reserve south of the Riviera Maya.

Odd critters such as the paca (kind of a spotted guinea pig on steroids), the coati, and the kinkajou (tree-dwelling raccoon kin), might pop up as you roam ancient Maya cities or hike through mangrove thickets. Though less often seen, jaguars, ocelots, margays, and other smaller cats roam the region's tropical forests.

Most of Mexico's 1,000 species of colorful tropical birds inhabit southern Mexico. The resplendent quetzal, which inspired the "plumed serpent" god of Maya legend, is among the world's most spectacular feathered creatures, with its shimmering, 2-foot-long tail. Its numbers have dwindled, but it can still be found in Chiapas' highlands.

Tabasco's wetlands are a paradise for bird-watchers. Crocodiles are also common, and you might spot howler monkeys, big cats, and manatees. Coastal waters throughout the region teem with dolphins, rays, and sea turtles.

And those are just the animals you might have heard of. Others in this region include the Mexican caecilian, a primitive amphibian resembling a half-meter-long (1 2/3-ft.) earthworm; the striped basilisk, called the "Jesus Christ lizard" for its ability to skip across the water's surface; the roseate spoonbill, a flamingo burdened with an elongated duck's bill; and other strange and marvelous creatures.

Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.