One of the Yucatán Peninsula’s most intriguing features is its cenotes, large freshwater sinkholes. Cenotes owe their formation to the massive meteorite that hit the Yucatán Peninsula 65 million years ago, near present-day Mérida. The impact shattered the peninsula’s thick limestone cap like a stone hitting a car windshield.

Over millions of years, rainwater seeped into the cracks, forming what is today the world’s largest underground river system. Cenotes are former caverns whose roofs collapsed. (Cave-ins are extremely rare today, however.) Cenotes can be hundreds of meters deep and are usually filled with fresh water (occasional seawater intrusion in some cenotes forms haloclines — a bizarre and interesting sight).

Cenotes were sacred to the Mayas, who relied on them for water. They also were seen as apertures to the underworld, and sacrificial victims were sometimes thrown into their eerie depths, along with finely worked stone and clay items. Archaeologists have learned a great deal about early Maya rituals by dredging cenotes near archaeological sites.

Still revered by many Mayas, the peninsula’s cenotes have attracted other worshippers: snorkelers and scuba divers.

The unbelievably clear water — 100-meter (328-foot) visibility in places — attracts many underwater enthusiasts. But the real joy of snorkeling or diving here comes from the amazing stalactite and stalagmite formations. Formed in the ice age when water levels were extremely low, they were submerged in water as the climate warmed. Today, you can snorkel and dive past beautiful formations that you would walk around in a dry cave.

Divers with open-water certification can dive in the cenotes. Though “full-cave” diving requires advanced training, most cenote tours are actually “cavern” dives, meaning you are always within 40 meters (130 feet) of an air pocket. It’s a good idea to take some open-water dives before your first cenote tour — buoyancy control is especially important in cenotes, and you’ll be contending with different weights and finning technique.

Here is a list of key cenotes, roughly arranged in north to south order along the Yucatán Peninsula. This list is by no means exhaustive.

• • Cenote Tza Ujun Kat, about 45 minutes from Mérida
• • Cenote de Cuzamá, about 45 minutes from Mérida
• • Sacred Cenote, near Chichén Itzá.
• • Cenote Sagrado Azul, near Chichén Itzá
• • Cenotes de Dzitnup, near Valladolid
• • Hidden Worlds Cenotes park, near Tulum.
• • Cenote Azul, near Laguna Bacalar