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Distance travelled ~ 709'020'800 km

Getting geared up for the dive

Sharks and stalactites may be close to each other in the dictionary, but you would think that reality keeps them a safe distance apart. For a start, sharks aren't known for inhabiting caves, and every stalactite I've ever seen has been in a cave. Secondly, stalactites can't grow underwater and sharks can't breathe if they're taken out of water. That sounds like a clinching argument if ever I heard one, but the thing I love about science is that our world is more complicated and interesting than that. Not only did I see lots of sharks swim past lots of stalactites this week, but this weird combination tells us something fundamental about our planet. And it's not that a flock of flying sharks has started spelunking because they suddenly fancied bats for dinner.

Belize is just next to Guatemala and south of Mexico, tucked into the back of the Caribbean sea. Its coastline is littered with islands and coral reefs, but what brought Jacques Cousteau here in 1970 is circular deep blue hole in the reef. We arrived in Belize last Monday laden with SCUBA gear, all ready to explore that hole.

Going into the hole was pretty eerie. There is sand and coral right up to the edge, and then the vertical wall just drops away into the darkness. We left all the brightness and light and colourful fish behind, and sank slowly. After going down a little way, all I could see was the rock wall stretching into the gloom. I found looking away from the wall a bit disconcerting because it felt as though anything could swim out of the black, even though I knew perfectly well how unlikely that was. We kept going down further and further, and I stared at the wall, straining to see what on earth brings people here. A reef shark swam past just two metres underneath me. And then the gloom readjusted itself just in front of me and I was looking at a stalactite that was nearly a metre wide at the top where I was, and was probably 5 metres long, pointing downwards into the depths. It was monstrous. There was an overhang, like an upside-down shelf a few metres deep, and looking along it I could see other stalactites hanging down, all of a similar size. We swam along the overhang, and the sharks cruised past us a few metres further out from the wall.

Dives that deep have to be short, and we had work to do, so it was only that night that the scale and the incongruity of what I'd seen sank in.

The size of the stalactites helps you understand the size of the story they're telling. Both are gigantic, almost too big to fit into a human brain. The reason that the stalactites are down there at all is that during ice ages, sea level gets much much lower. 15,000 years ago, the last time those stalactites were growing, they were on a cliff in dry air because sea level was 120 metres lower than it is today. That's the sort of fact that you can read and understand logically, and it's something that I had known for years, but it's hard to digest properly. Read it again: 120 metres lower. That is an awful lot of ocean that wasn't there. Floating in the darkness with 40 metres of water above me, next to a rock wall that kept going downwards as far as I could see, I came closer than I ever have to really understanding the enormity of the changes that ice ages bring to Earth. Oh yeah, and there were sharks too.

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