It's not the darkness. It's not the tight squeezes. It's not the sharp rock, the damp or the fact that I'm so far off the grid. No, it's the fact that each step forward reveals more and more bones, scattered and broken by time, but unmistakably human.
Skulls with hollow eye sockets stare right through me. Some are missing jaws, some have huge cracks where a weapon of some sort did its damage. Whoever these people were, their deaths were almost certainly violent and their bones do not rest in peace.
The Burial Cave of Rimarua, on the island of Atiu, is unusual for a number of reasons.
First, Atiu is part of the Cook Islands, a place more associated with honeymoons, hammocks and dreamy turquoise water than old bones. Secondly, as far as I know, there are no other burial caves in the Cooks, but if you do find one, good luck getting permission to visit it. Third, very little is actually known about Rimarua, as no formal archaeological studies have been undertaken. All of which leaves us with a decidedly unique - and happily legal - opportunity to explore an ancient cave packed with the remains of fearsome Polynesian warriors.
Marshall Humphries, an Englishman who has gone "troppo", has been leading tours to the burial cave for years. Returning to the island with his Atiu-born wife, Marshall has rare permission from the landowners to take tourists into the cave, parts of which he continues to explore with his kids.
The island itself is volcanic, protected by a reef and coral limestone cliffs. Captain Cook was the first European to explore it in 1777, welcomed by locals in much the same way I was welcomed by its current tribal chiefs - with a coconut cup of homebrew in a traditional tumunu, or bush pub.
Atiu Islanders are an exceptionally friendly bunch, eager to show visitors the natural beauty and wonders of their island. I attend a local birthday party, eat sweet papaya for breakfast and star-gaze in the warm, breezy night.
But things were not always so friendly. Atiu legend tells of a time when warriors from another island had the misfortune to underestimate an attack on the locals. After a massacre, their bodies were dumped into a hole in the ground.
Centuries later, I was carefully lowering myself into this very hole, using slippery tree roots as footholds. Marshall admits this adventure is not for everyone - especially the overweight and claustrophobic. Within seconds, I'm muddied and ducking under low rock. Avoiding sharp coral limestone overhangs, I make my way further into the dark, my headlamp lighting up piles of skulls and bones.
"There's more over here," says Marshall. "And here, and here." Since nobody has studied the cave or its history, nobody knows how old the bones actually are. "Mind you don't step on any skulls," instructs Marshall cheerily, as if it is the most natural thing to say in the world.
I slide on my belly down one particular shaft and find yet more bones, swept into a pile by heavy rain over the years.
I find myself face-to-face with a skull, sporting a large hole in the cranium, the fatal mark of a Stone-Age weapon. Could either of us have ever imagined we would meet across time, space and culture?
I squeeze around, slither up the shaft and make my way back to the cave entrance. Mosquitoes are waiting in ambush, so we jump in the pickup and head straight for a tumunu, buzzing with the gentle sounds of ukuleles.
An ancient burial cave on a beautiful island in the South Pacific? Together with whatever the chief puts in his homemade hooch, it adds up to a sure-fire recipe for feeling alive.
Getting there: Air New Zealand operates up to eight direct flights a week from Auckland to the Cook Islands and one direct flight a week seasonally from Christchurch. Air Rarotonga operates links from Rarotonga to Atiu and Aitutaki.
What to do: The Rimarua Burial Cave tour lasts 1.5 hours and some physical restrictions apply. Prepare to get really dirty and bring insect repellent. Price is $25 per person. For more information see here.
Atiu's other attractions include birdwatching, snorkelling and bushwalking.
Further information: For general information about visiting the Cook Islands click the link.
Robin Esrock was a guest of Cook Islands Tourism.