IT’S 16 feet down. ‘Only as high as the first floor of our house,’ yells my husband, encouragingly. But to me, peering from the cliff just round the corner from Dancing Ledge, it feels as if I am about to launch myself off the Empire State Building.

Twice I take a deep breath. And then shrink back into the cloud-grey rock as Eddy Young, whose company, Dorset Adventures, is guiding us, talks soothingly about what I have to do.

“Pick a point on the horizon; don’t look down,” says Eddy gently, as my legs turn to jelly.

Even as the words are coming out of his mouth I have done it – stepped out over the briny and plunged into the cool, clear waters.

First thought: it’s a long way down. Second: half the English Channel has gone up my nose.

I forgot to breathe out as I went in. I also stepped in the wrong direction but that doesn’t matter, as Eddy and his instructors have checked out this entire strip for safety, which is why coasteering is so different to the discredited practice of tombstoning. And I am wearing a wetsuit, trainers, helmet and a buoyancy aid.

Coasteering was invented, says Eddy, in the early 1960s, probably by experienced climbers like him who: “Started trying to gain access to bigger, harder routes down by sea-level, by traversing or doing a bit of swimming access. It wasn’t an official activity or sport as such, they were just using it to further their climbing adventures,” he says.

Now it’s one of Britain’s fastest-growing recreations and it’s not hard to see why; it’s interesting, ecologically-sound and enables the cissy-of-tendency (people like me) to test themselves without fear of it all going wrong. Challenge by choice, Eddy calls it, which means there is no goading, no pressure and no shame if you decide there’s any aspect of the journey you don’t want to do.

“What I want more than anything is for people to enjoy it,” he says.

We change in Dancing Ledge car-park and Eddy, me, my husband and instructor Tom walk the track and down the steep hill to the Ledge itself, Eddy explaining bits and pieces of safety along the way.

We’re told about safe exit and entry points, and also about the two rendezvous places along the way, vital for when he’s taking the bigger groups like the Stags and Hens who frequently choose this activity for their party.

Eddy explains how Dancing Ledge was formed by the quarrymen who blasted out the now-famous pool. He points out the worn stone rail-tracks and explains the terrain we’ll be going over.

We start off with a little light sea entry. The first is a kind of skydive position. Then it’s jumping into the famous pool and practising our swim-round.

We’re on the intermediate course but, Eddy says, he evaluates from the first 10 minutes how able his clients are.

“Safety is our absolute priority,” he says.

“I’m fully qualified and insured, as are the assistants.”

As a member of the RNLI’s National Water Safety Forum’s beach advisory group he helps maintain good practice for coasteering and provides guidance for those who want to train as leaders in this sport.

Tom leads us across a pale limestone ledge, the waves lapping round, all the time warning about the sudden holes and crevices that can grasp the unwary foot.

After a few more four-foot jumps we’re off again, with a gentle climb across some rocks to the place where we’ll do our first ‘big’ jump. At first even eight foot seems scary but it goes really well.

“This will seem like nothing after the big one,” Tom promises.

Eddy’s route takes us into a cave, which I had been dreading, fearing a slimy black hole, filled with voracious sea-monsters. But no. The cave is as big as my garage, considerably nicer inside and is filled with slapping blue water, sliced through with sunlight. We swim for the back and creep round a rock into the next space where we spend several happy minutes being swashed around by the waves, while Eddy points out a giant ammonite embedded in the roof.

At this point we could do what he calls ‘the sump’ a half-submerged arch leading into the next cave but instead we choose to swim on round.

The route leaves plenty of time to just enjoy, so we spend a few minutes bobbing on our backs, watching a cormorant and his wife, tending to their nest. Because we are quiet and don’t threaten, they are quite happy to just sit there, watching us. Eddy explains that regard for the natural environment is one of coasteering’s important aspects.

“Because we are using the same area as nesting gulls, we’ll make sure we don’t disturb them and that means that routes can vary slightly over the course of the year,” he says.

His point is proved when, without warning, a Rib boat powers up to the next inlet and suddenly a flock of startled birds takes to the air.

I recognise them straight away – they are the only puffin colony on the south coast, and one of the things I have always wanted to see in this area, but it’s sad to see them so apparently distressed.

In direct contrast, our cormorants are completely unruffled by our presence.

“Sometimes,” says Eddy, “You even get to swim with seals.”

It’s a bonus we don’t get but the day has been truly fantastic; fascinating and fun. I could get addicted to this...

• Coasteering with dorset-adventures.co.uk takes place in groups of six or more