Dom Joly: Welcome to my glass menagerie

I'm in Indonesia, on the island of Sulawesi, on a two-week trip to teach my daughter to scuba dive. I have a friend, Kaj, who is a scuba dive master and, whenever he moves to a new job we go and visit him. "You'll love this new place," he told me on Skype. "It's world-famous for muck diving and there are loads of weird creatures totally unique to here."

I wasn't quite sure what muck diving was –it didn't sound enormously appealing, but a trip anywhere that gets me out of freezing Britain is a green light to me. So a day and a half later, after an overnight stop in Singapore, we arrived on the northern tip of Sulawesi, a Christian enclave in this, the largest Muslim nation in the world. Our destination was the Straits of Lembeh, a narrow channel between Lembeh and Sulawesi. This, it turns out, is a renowned destination for divers, because of its "critters".

This was exciting. I adore diving, and the idea of cavorting once again with giant turtles, sharks and dolphins got me all warm and fuzzy. But it was not to be. It turns out that the Straits of Lembeh are famous for macro-diving. This is the pursuit of some seriously tiny creatures – some no bigger than a human hair. It's one of the annoying things about the sport – the more they dive, the more interested people seem to be in smaller and smaller stuff. I love seeing really big stuff, feeling that frisson as a huge ray glides past you or floating, as still as possible, while a shark sniffs out some lunch. This sort of thrill, however, is old hat for experienced divers who come to Lembeh for something a bit different.

On my first dive, my companions were raving about a piece of yellow coral. They crowded round it, taking photographs and indicating that I seriously needed to take a look. I did, but couldn't really see what all the fuss was about. I've seen loads of coral around the world and it would take more than this to get me worked up.

I swam on, and spent the rest of the dive watching an octopus that had made his home in a discarded coconut shell. When we eventually surfaced and were all swapping stories on the boat, I was pretty sure that my octopus was the story of the day. I was wrong. Everyone was on about the yellow coral. "What is the big deal with this coral?" I asked. "Didn't you see them?" asked my companions, looking dismayed. "What?" I asked. "The pygmy seahorses," they replied breathlessly. "There were three of them. They were incredible."

One of them produced his video camera and replayed the sighting. I still couldn't see anything apart from the coral. "They're really tiny. They were only recently discovered by a scientist who took a small piece of coral back to his lab for analysis and found a dead one under his microscope." This was clearly going to be a tricky trip for me. I am getting long-sighted and need reading glasses for a newspaper, let alone pygmy seahorses. Kaj had a solution: "You could get a mask with prescription lenses, but for now...", and he handed me a huge magnifying glass.

Now as I cruise the underwater world, I occasionally produce my huge glass and peer at some very unnerved sea creature. Kaj has a special video camera with a magnifying lens. Every night he shows us footage of unbelievable critters that truly defy the imagination. Just as you're marvelling at them, a human finger appears in shot, completely dwarfing the subjects and I realise that I'm going to have great difficulty in spotting anything. It's not easy getting older.