“This will be your first physical challenge,” the guide told us as he pointed his flashlight at a hole no bigger than a kitchen sink where the cave wall met the floor. I thought this was coincidental and he was referring to a steep climb ahead, or a slippery descent. But he kept the light aimed at the opening.

I gazed at it again. It looked like something a hideous subterranean rodent might crawl out of. It went about 30 degrees downwards, and had so many pointed edges that the strong LED lamp couldn’t illuminate beyond a metre in.

“If you can handle this you can handle everything else,” he said. Then he asked for a volunteer to go first.

We were two couples, the other one a doll-faced girl from Cúcuta named Karen and Paul, her tall English boyfriend with a shaved head and not a word of Spanish to his lexicon. “I’ll go,” I said and eased my feet in, facing up.

If cavemen played limbo, it would have been something like this. I had to keep my body erect as my feet felt for something to step on and then slowly shimmy my butt onto any surface that would hold it. I couldn’t look down to aim my helmet lamp at where I was going. The edges of the rock grazed my face and at one point I had to raise one arm, as I would not fit with both at my side.

I felt like I was in The Thing’s birth canal.

He had a point. Up to this moment none of our challenges had been very physical. About three minutes into the cave, when no more natural light crept in, I felt a slight burn in my nostrils, the familiar tang of bleach.

“What you’re smelling is ammonia, which is a component of guano,” the guide said.

Guano. Bat shit.

Bats.

“It’s okay, the vampire bats feed mostly on cattle blood,” he said to assuage us. We silently agreed he could have spared the word “mostly”. The squeaks and the flapping wings grew louder and the guide pointed his flashlight at the first nest. It was a writhing mass of grey on the cave ceiling. I was reminded of that scene in Matrix 3 where the machines break into the human citadel.

Just below the nest the rocks were spattered with a hardened black ooze with white dots. “The bats just extract whatever nutrients they need from the blood and then secrete the rest,” our guide said. Though bats are blind, they are nonetheless sensitive to light, and every time we shone a light at a hanging cluster, several took flight, sometimes right over our heads.

The girls did a good job of holding back their shrieks. They couldn’t stop associating them with vampires. I thought of Batman, and it made each subsequent colony a bit more optimistic.

Yes, you could say that was our mental challenge. And then there was the clay corridor, which tested both. “This part is optional,” the guide said, which was to me and Paul was a blatant dare. We squeezed through a narrow horizontal fissure and found ourselves ankle-deep in soft, slimy mud. My sandals became stuck to the clay like a molar on a dense caramel. After a few steps, the mud felt good, like we were walking on a delicious creamy dessert.

And when we reunited at a spacious chamber, the guide asked us to turn off our lamps and sit in complete silence for five minutes. It was utterly black and all we heard was the occasional drop from a growing stalactite. And suddenly it occurred to me. Not once in this trip, not during the cave visit or the rafting earlier that day, was I asked to sign a legal release form. They didn’t ask me to check off any ailments that could make me less than fit, or acknowledge that there are risks involved, including, but not limited to injury, drowning, hypothermia, coma or death. And I was never asked to relieve them of any legal liability.

The agreement was implicit in the activity: It’s a cave. It’s dark and there are rocks. I chose to come here. In a way, that first grueling hole was our release form. If I wasn’t aware of my own limitations, I had only myself to blame if something went wrong.

“If you can handle this you can handle everything else,” the guide had said and he was absolutely right.

I found this incredibly civilized.