Tak Be Ha is one of many windows into the land of the dead. From here we may enter the first of the nine layers that make up Xibalba, the ‘place of fear’, the Mayan realm of the underworld.

We climb down through a narrow opening in the ground and find a wooden ladder in the subterranean gloom. After the heat and dust of Mexican midday, the cool darkness is a shock, we shiver away on a rocky platform. Then we must submerge ourselves into the cold clear waters below, sinking down under the surface, ritually dying.

From above the water looks flat and still, a milky azure, shimmering gently in the darkness of the cave. Once we dive under the surface though a dramatic jagged landscape of underwater mountains and crevasses suddenly appears. There are stalagmites and columns lit up by hidden phosphorous lights. Deep ravines fade into midnight black far below us. The underwater world here is far wider and more expansive than the size of the cave above should allow. The limestone walls slope far away beyond the upper chamber and they seem to go down for ever. But beware! If you explore too far outwards you can’t surface for air – the roof traps you underwater. Spectral wraiths slither out from caves far below us in the darkness, lit up by underwater lanterns. We think they are scuba divers silently exploring the deeps, but who knows?

Tak Be Ha: syncopated hard syllables that evoke a brutal ancient language, Tak, a chopping sound. Bay, the exhale cut short. Haaa, a whispering sigh as the soul is released. There would have been sacrifices here without a doubt. There must be bones, daggers, gold, ceremonial masks lying undisturbed down in those dark crevasses, but we are only equipped with snorkels and goggles, so we float on top of the world of the dead. Our time has not come. We will not journey to the deeper layers.

This is one of many freshwater sinkholes in this part of the world – one of thousands even. Some are as large as lakes and open to the skies while others are just holes in the vegetation, narrow shafts with subterranean chambers branching out from them. There is no standing or running water on the surface of the Yucatan peninsula, no rivers or lakes, instead rain has dissolved away the limestone over centuries, and the water table permeates below the surface in underground chambers and channels that riddle the bedrock.

The Mayans named these cenotes, ‘sacred wells’. They were not just a vital source of water but a crossroads between the worlds of living and dead. Here at the gateway to the spirit world Mayans could communicate with their gods, perform religious ceremonies, carry out burials and conduct sacrifices.

We have already visited Cenote Azul, a pleasant system of outdoor swimming holes, rock jumps and picnic spots where fish nibble your toes as you dangle them in the water. It was well-run and the entrance fee was reasonable. We saw none of the gods amid the nicely maintained wooden walkways and recycle points but that doesn’t mean they weren’t there. Tak Be Ha is a very different experience though. We have to find our inner Indiana Jones as we enter. The underwater landscape is scary but spectacular and for hours we flap our way through underwater caves and limestone labyrinths, Matilda’s little white legs writhing like underwater snakes ahead of me in the turquoise gloom.

We are very into Mayan mythology at the moment. We have already ritually sacrificed Matilda on a stone altar to Ix Chil, the moon goddess. This was high on the cliffs in the Mayan ruins that overlook Tulum, the city we call home this week. Now I am wondering if we need to make another offering to Yum Cimil, the Lord of Death, down in this subterranean cave. Arthur would be the obvious choice.

In the end we risk the gods’ displeasure and return to Tulum with a full car, even squeezing in three hitch hikers, for somehow Avis has sent us off with a minibus instead of the SUV we ordered.

We are enchanted by the magic of the underworld though – silent, cavernous, cool and blue, a sanctuary away from the heat, dust and searing light up on the surface – and so we end up seeking out a second cenote in the afternoon. Cenote Calavera is a single bat-filled cave sitting in a stretch of jungle just outside Tulum. To enter the cavern one must jump through one of three holes in the ground and plummet down into the water below. The drop is about five meters. It is a leap into the dark in the truest sense.

It takes Matilda a long time to pluck up the courage to do this, but once she is initiated, the magnetic lure of the deep is established. She and Arthur spend the whole afternoon throwing themselves into the fathomless waters time and time again, trying ever wilder jumps and dives.

There is a hysterical American lady who takes on the role of sacrificial victim. She spends half an hour hesitating, moaning, unable to pluck up the courage to jump (“Um gonna do it! Um gonna do it! Oh gawd, uh can’t!). Various onlookers scattered around are chanting and cheering her on. Roll back the centuries Oh Ixtab!

Matilda appears beside her to offer advice, an ephemeral spirit-guide in a pink swimming costume, showing how one might enter the dark waters of Xibalba with a neat pencil dive. She has become emissary to Camazotz the Mayan bat monster I think. I see it in her red rimmed eyes, those sharp little teeth, that insatiable yearning for blood.

It is a deep and mystical week we pass in Tulum. We are steeped in legends of bloodshed and sacrifice, of psychedelic gods and shamanic rituals. We visit the incredible Mayan ruins at Chichen Itza and sit before the great pyramid that was built in homage to Kulkukan the God of the Wind. Then onwards we go, to the nearby Cenote of Ik Kil, which is not wild and spiritual at all, but run like a soviet swimming pool complex. At its heart is a deep well shaft that has been reinforced by concrete walls where trailing creepers hang, so we feels like we are floating in a giant abandoned nuclear reactor.

We visit next day the cenote of the Dos Ojos, two pools each with a central island that is ringed with dark glittering water like kohl-stained tearful eyes. We commune with the dead one final time here and then we come back to the surface and walk away. We are done with cenotes.

We go back for a last night out in Tulum before we move onwards. Menna and I flirt with Acan, the Mayan god of intoxication.