No, this isn't an advert for a credit card or the opening to a rum '50s crime novel, but a story about a hobby that captivates the imagination of 38,000 people in Britain – scuba diving.

I was diving around in the city's outdoor pool because the city's branch of the British Sub-Aqua Club is currently looking for new members, and is offering people the chance to go for a £10 "Try Dive."

This is where an experienced instructor will kit you out and take you underwater, even if the only experience you have of diving is searching for your rubber duck in the bath.

But why does anyone bother? Aren't there enough sports to enjoy on dry land without having to scrape the sea bed as well? What's the big deal about diving?

"Simple," said diver and instructor Peter Darling. "Diving is great because you get to see and experience things that other people just won't ever see or do."

Because although the city's sub-aqua group meets every Monday at the Lido, that is only for a get-together and training sessions.

What they really enjoy doing is heading out to the eastern coast of a weekend, having a good old dive, and peeling back the layers of history hidden in Davy Jones's locker.

"This area of the coast has numerous ship wrecks, and we have even discovered a new uncharted wreck," said Peter, who works for the Office of Rail Regulation.

"One of our members was looking through the records of Cromer Life Boat, when he noticed they had been called out to a Spanish boat calling 'mayday' during the '30s, at the time of the Spanish Civil War.

"But this wreck wasn't on a map, and so we had to look for it. We knew the very rough location, but it was really just like looking for a needle in a haystack. It took us all summer to find it, and then we did.

"But by then the weather wasn't suitable for us to dive in – the sea gets very choppy during winter which makes it hard for us to dive, and so we don't. But we went back there first thing the next summer.

"We didn't mind the wait because when you're looking for an uncharted wreck you know you're not exactly in for a five-minute job.

"And when we got down there, it felt like finding the Titanic. It was absolutely stunning."

Often, wrecks are "swept" by the navy, meaning the top of them is crushed so nothing sticks out to be dangerous to passing ships. But the boat Peterborough's divers found hadn't been swept, and was in perfect nick.

"We swam all around it, but didn't swim in it because that's dangerous," said the club's chairman Robin Clark (35). "Seeing wrecks is always exciting because it's something that normal people will never see," added Peter. "It's like nothing else.

"But as a mark of respect, divers will stay away from wrecks on which people have died. There is a war grave in Scapa Flow, where a German U-boat fired a torpedo and hit a ship, and about 800 people died. All divers know where these places are and stay away from them."

Spooky – but interesting. Also interesting is the idea that diving reminds humans of where they began – in the womb.