With a map, compass and machete, you might hack your way there in a day or two. Miss it by just a few yards, however, and the thick, tropical foliage will hide any evidence of its existence. Who knows how many of these clear-water oases perforate the limestone substrate of Mexico's Yucatan Peninsula? Thousands are documented, and many more lie waiting to be rediscovered by modern men. The Yucatec Maya called them cenotes, water windows to the underworld of Xibalba. The gods of their mythology traveled through the dark, water-filled passages of Xibalba using cenotes as their exits to the world above. Certainly, I am not the first diver to fall under the spell of the Yucatan underworld. Even jaded reef veterans and grizzled wreck hunters are humbled when they first slip beneath the inviting blue-green water of a jungle cenote to witness the play of sunlight and shadows in the cathedral-like grottoes below.Cave to Cavern The first wave of cave explorers descended on the Yucatan in the mid-1980s. Lured by tales of warm, clear waters, shallow depths and extensive, intricately decorated cave passages, these pioneers blazed trails deep into the jungle. Accessing a new cenote often required a sweat-soaked hike on a narrow jungle track, followed by a harrowing climb down craggy limestone cliffs. The payoff came in the form of miles of virgin cave passage that included some of the world's most spectacular underwater geology. Many of the Yucatan's caves were dry during the last Ice Age and remain heavily and intricately decorated with stalactites, stalagmites and flowstone. For divers accustomed to the stark limestone conduits of Florida springs, Yucatan cenotes seem less like the underworld than a water-filled heaven on earth. Stories of 10-mile passages and photos of divers suspended in white-walled, formation-adorned chambers soon enticed another wave of cavers. These second-generation divers came to enjoy rather than explore, and as their numbers increased, cenote diving evolved into a profitable business. Landowners asked for access fees of a few dollars and in exchange built trails and then roads to access the more popular cenotes. Simple but functional ladders, stairs and boardwalks were added to facilitate the access, and jungle thickets were transformed into park-like settings. Because cave divers are a rare breed - even in the Yucatan - local operators soon turned their attention to the hordes of open-water enthusiasts traveling to the nearby resorts of Cancun and Cozumel. Soon, dozens of guides offered cenote tours to open-water divers. Fortunately, peer pressure and guidance from U.S. cave-diving associations helped prevent the tragic accidents that plagued the early days of Florida cave diving. Permanent guidelines are now installed in the popular caverns, and dive guides know and respect the special requirements of diving within an overhead environment. Another factor working in favor of Yucatan divers is the sheer scope of the caverns. Unlike lava tubes, coral grottoes and spring outflows, Yucatan caverns are not merely shadowy entrances to tunnel-like passages, but huge, light-filled rooms. Many of the region's best caverns are actually the remains of mammoth underground chambers exposed to the sky when their ceilings collapsed. By skirting the edges of these collapsed and sometimes connecting rooms, cavern divers can swim a mile or more while remaining in the twilight zone that separates open water from the dark cave passages beyond. Cavern diving continues to grow in popularity, especially among divers who split a week between the reefs of Cozumel and the mainland cenotes, which are only two hours away by ferry and shuttle bus. Just as they do at the springs of Florida, open-water enthusiasts now outnumber cave divers at the more popular cenotes, and guided dives and cavern/cave instruction flourish.Diving the Edge In the course of a week, we visit many remarkable caverns. One day, we climb aboard a jungle buggy that resembles something out of Mel Gibson's Road Warrior films, bounce our way down a rutted jungle track, and lower our gear and ourselves through a manhole-size opening into a semi-flooded underground room. Welcome to the Yucatan. The same day, and only a few hundred yards away, a group of German divers arrives in an air-conditioned van; they assemble their gear on shaded benches, then stroll to the edge of the water to begin a two-hour cavern tour of the spectacular Dos Ojos system. Because our cave and their cavern connect by way of a short passage, each of us enjoys the same sights - we just arrive in different styles. And that, I discover, is one of the primary attractions of Yucatan diving. It can be as wild as a jungle hike to a hidden cenote, with pack horses carrying the gear, or as civilized as a van ride to the water, followed by a lunch of sizzling fajitas at a beachfront palapa. Our dives are as varied as the cenotes themselves. Some days we fin through large, stalactite-encrusted rooms; other times we drift in sun-dappled pools of green-blue water. It is on the last day of our trip that my vocalized thoughts turn to Xibalba. We swim the B line, as planned, and enjoy another phenomenal cave dive. Next, we locate the heavy-duty gold-colored line installed for the benefit of cavern divers. At the far end of the pool, the line dips deeper into the darkness of a wide passage. Just as the last bit of daylight fades behind, I catch a faint glimmer of blue light ahead. We round a corner just as the sun comes from behind a cloud, sending a laser-like shaft of light through a small hole in the roof of the chamber. We hang motionless and weightless in a semi-flooded underground room the size of an auditorium. The single shaft of sunlight reflects and refracts off the water's surface, lighting the far walls with momentary blasts of blue radiance. We surface and, for once, are speechless.