Cave diving is like swimming through the history of the planet. There are remains of both humans and animals but also stalactites and stalagmites. These cannot form when the cave is flooded, so you can see when parts of it were submerged and when it was dry.

Yet when I’m in a cave, time does not tick. There is no natural light, so the cave looks the same, whether it’s midday or midnight. If you cave dive without the right training, equipment and mindset, it can be a very dangerous place. I have a very meditative focus when I’m down there. I live in the now. I cannot think about anything else but what is happening in the cave. I find that very soothing and relaxing.

Caves are intrinsically connected with ecosystems and the lives of the people that live around them. They are among the places that suffer the most – out of sight and out of mind. In the Bahamas, water flows to the cave from the land above, transferring nutrients but also pollution. In this way, discharges from, say, a factory on the surface can move through limestone to the cave and affect entire seabeds, mangroves and rivers.

The closest environment we have to space is in caves. Some of the caves I dive in are hundreds of thousands of years old and the marine life there is unique: animals such as blind crustaceans and fish, which have adapted to live in eternal darkness and very low oxygen environments. These are animals that, to me, seem as alien as can be – yet we spend trillions of dollars exploring space. Why are we going to Mars when these animals are right below our feet?

A lot of the work I do involves collecting data for conservation and scientific purposes, and digitally mapping the caves so that people can see where a cave is, relative to the land above. This is very important as part of conservation efforts to limit land development.

You need to be an experienced diver to become a cave diver. I have done more than 5,000 cave dives in 80 or 90 different cave systems, and am now a cave explorer, which means I have found virgin caves. I also teach cave diving at the most advanced levels. If I don’t dive in a cave for two or three days, I miss it so much that my husband can tell. He looks at me and says: “you need to go diving”.

Sometimes, sharks hang around: you’ll see silkies, lemon or Caribbean reef sharks because there are a lot of nutrients and fish. Nurse sharks also sometimes sleep right at the entrance of the ocean blue holes.

I swim right past them – I’m as comfortable with sharks as I am with caves. They live in the ocean, this is their environment and I am just a visitor.

I often feel immensely privileged to be able to do what I do, although sometimes, I must admit, I forget how unique the world of the cave is, because it’s so much a part of my daily routine. But then I’ll look at a column of a stalagmite that’s 50ft high and wide enough to park a car on, knowing that it was constructed drip by drip, by an inch-and-a-half a year, and it will blow my mind.

• Cristina Zenato is a Padi course director and advanced cave diving instructor. As told to Donna Ferguson