Into the abyss

Bonito: It’s while I’m dangling on a rope through a crack in the earth more than 70 metres above an underground lake that I begin to seriously question my sanity – not for the first time on this journey. The lake is more than 80 metres deep but from this height, if my fastenings fail, the impact with the water’s surface will kill me instantly.

Nick is definitely more comfortable with our slow slide down to oblivion but because we’ve been ‘safety-tied’ together, I make him wait for me while I nervously release the rope brake. We had rappelling lessons last night on a gym wall, but that was easy peasy – a short few metres in a nicely lit hall before we nonchalantly signed our lives away. Hanging on a rope through a fissure in the rock the height of a 25-storey-building turns out to be a rather different experience. We’re halfway down when I notice a disquieting burning smell coming from the pulley I’m gripping as the rope slides through. Nick notices it too.

Maybe it’s the friction gloves we’re wearing, I say hopefully. No, Nick confirms, it’s definitely the rope. Yikes.

We reach the bottom eventually and stand in a vast subterranean cave on a small wooden platform that floats on the lake. It’s eerily beautiful and once our eyes adjust to what little light penetrates from the thin shaft, we begin to make out stalagmites and stalactites decorating every portion. This is the Anhumas Abyss, a natural architectural wonder formed over millions of years from rain slowly eroding the limestone here and building up crystals of calcium carbonate.

We tour the cavern by torchlight, finding a curtain of folded limestone, chandeliers of rock that occasionally break off (we wear helmets throughout) and monoliths decorated with crystalline rosettes.

The water here is incredibly clear (to 50 metres in summer sunlight) and peering down past the reflections of stalactites we see their opposites growing from the cave floor. These are the world’s biggest cave stalagmites – the tallest is 19.5 metres high – and we change quickly into wetsuits, eager to see them up close.

The water is freezing cold, clear and dark. Shining our dive torches produces a bright spotlight that shrouds everything else in deeper gloom, so I keep it off and let my eyes welcome in the edges. We float 20 metres down through golden cones of stalagmites, trying not to bump them and disturb their perfect preservation. An anteater skeleton lies where it died, stumbling over the hole’s edge 100 metres up.

There are small fish called lambari here in this disconnected pool of water, grown from eggs dropped by birds. They eat the little algae that grows here and hungrily feast on our dead skin cells. Below 30 metres, tiny shrimp inhabit the waters, swimming out of the range of the fish.

The cave was only discovered a few decades ago when firefighters putting out a bush blaze here revealed the crack. The soluble limestone here makes for many caves and there are more than 20 in the 30 kilometre radius of Bonito, a small town in the southern Pantanal. The Anhumas cave was first explored in 1984 by geologists who were astonished at the scale of the stalagmites. Tours here began a decade later, presumably once they’d worked out how to get tourists in and out without dropping them.

We ascend the lake and then prepare for a greater ascent: out of the cave. Climbing 72 metres (236 feet) out of this subterranean hole is not easy. We do it by an exhausting method of hauling ourselves up with our arms, using the rappel grips, and then pushing down with our legs. It’s a slow, painful process that takes the fastest experienced guide an impossible 3mins 40secs, but takes us more than 20 minutes.

Up and out, back in the warm sunlight, it feels as if we have emerged from a different world.