Cenote Dos Ojos

A few wooden steps take you into what looks like a swimming pool with a rock lid. The floor at the entry is pale sand, the water is an almost translucent blue, and for the first minute nothing about it reads as a cave. The dark starts at the edges, where the ceiling comes down to meet the water. The water is 24 to 25 C in every month.

The Barbie Line goes first. It runs out along the opening of the second eye, roughly 415 m of permanent line with daylight on it from end to end, and room to swim around big columns and stalactites under shafts of sun dropping through the openings. Starting in bright water shows how much faster buoyancy moves in fresh water than in the sea.

Then an hour on the surface, and the second dive has a different mood. The Bat Cave line heads west, about 300 m into much less light and a few metres deeper, through smaller chambers whose roof has not collapsed and whose formations are better kept. The enclosure builds as you go.

The high point of that route is an ascent. The line passes an air-filled dome and the guide signals the group up into it, and you break the surface in a chamber under the jungle with a colony of bats clinging to the limestone above you. The stay is around six minutes, about halfway through the dive. Then it is back through the darker sections to the steps.

Two different dives start from the same set of wooden steps, and the surface interval between them is spent at the park. A cenote day usually means diving one site, packing up and driving to the next; here the second dive begins at the pool where the first ended, so the whole day runs from a single entry.

The name is literal. The two eyes are neighbouring sinkholes joined underwater through a larger third cavern, with roughly 400 m of passage between them, and both routes start and finish in the first eye. One route is wide, bright and long-sighted, with columns and stalactites under open shafts of sun; the other is tighter and darker and ends inside a chamber of bats.

The light is also a matter of season. From May to September the sun stands high enough to send shafts down through the openings on the Barbie Line, and the park is busiest from November to April, when the afternoon slot is the quieter one.

Cave divers began exploring through these two sinkholes in the late 1980s, and the work has not stopped. For three decades the cave behind them was known as Sistema Dos Ojos and counted among the longest surveyed underwater caves in the world. An underwater connection to Sistema Sac Actun turned up in January 2018, and by survey convention the smaller system took the larger one's name. Sac Actun is surveyed at 379,569 m of passage with no Sistema Dos Ojos beside it,1 so any system length still printed under the old name describes a particular year.

The Yucatán's underwater caves reached cinemas in Journey into Amazing Caves, the highest grossing giant screen documentary of 2001, which followed two cavers through ice caves, those caves and the Little Grand Canyon.2 On 3 November 2010 the Venezuelan freediver Carlos Coste swam 150 m through the cave on a single breath in 2 minutes 32 seconds, reported at the time as the first freediving record attempted inside underwater caves.3

The guide leads both routes, and the rules come from the landowner's gate and the regional standard CREER publishes: four divers to a guide, minimum age 15, a permanent line on both routes. Gate hours are listed as 08:00 to 17:00 with a last entry at 15:00, so a late arrival can close the day before it starts.

The Parque Dos Ojos ticket counter takes pesos in cash only, and the fee sits apart from the tour price. A camera adds 300 to 600, and some operators fold the gate fee into the quote while others add it, which explains much of the spread between prices. Repellent and sunscreen stay in the car.

Stay within arm's reach of the line on both routes, never pull on it or cross under it, and decline any offer to leave it.