The most surprising thing was the sound. As soon as we descended, I started to hear it. A high-pitched ring. I put my fingers in my ear, and tried to shake it out.

It was our Saturday dive with the rust bucket – and at Frank’s site. It is such a beautiful site, which dive instructor Frank showed us with Bonnie. Stuffed with mature coral, soft and hard. Situated in a part of the island situated away from large surge and storm events. Absolutely full of the coolest critters you can find on the protected side of Bonaire.

At this point I should reveal we didn’t have a camera. Well, I did lose mine after that deep dive with Jack and Lu a few weeks ago. It looks like it took a bath. But the GoPro which had taken such lovely footage of the shark the week before – was down to 7% and uncharged by Saturday morning.

‘Oh well,’ I said to Jack, ‘after last week’s shark, what are you going top that with?’ Doh.

We went along on our sides, admiring the coral. It looks like underwater trees and bushes – not colonies of ancient animals bound together in complex gorgonians. Like the above world has transformed into the below world. Stunning – and a little surreal.

Until, dammit, that ringing again. It went right through me. This time, I called down to Lu – a few metres below. Whatever you try to say with a regulator in your mouth sounds the same. Urghurghurghurgh! A bit like that.

But Lu looked back at me, and when I pointed at my ear, she pointed at hers too. Jack and Delphine were slightly ahead. Still, that was enough. I wasn’t the only one hearing it. And it got louder.

For about ten minutes, the water began to fill with squeaks, clicks and whistles. It was spooky, the strength of the noise. It makes you realise how much of the underwater world is powered by sound. The way it literally goes right through you, vibrating through the cells of your body. It felt like really intense bird song playing through an amplifier. And you’re standing next to it.

We stopped when it got really strong and just hovered over the reef. The parrotfish carried on as normal. The coral kept waving. Nothing. Just blue.

When they did come, they were finally silent. That was a cool trick. And our eyes kicked in, which also made us stop in our tracks. Ten dolphins. The largest about two metres long and quite uniform in colour. They glided slowly above us, a couple of metres away, eyeballing us as they went.

Each adult dolphin too was paired off with a smaller, juvenile dolphin. At least one of them seemed by its small size to be a proper dolphin baby. It seemed to hug its mum as they went past above us. Stomach to stomach as if riding in her jet-stream.

And of course no camera – so we couldn’t catch the image. The noise is still ringing in us.