Nice morning for an underwater walk

It was never going to be a walk in the park.

An adventurer who plans to walk underwater from Robben Island to the mainland at Cape Town has stumbled on an uncomfortable truth about Table Bay.

London business manager Matthew Silver-Vallance has already made a name for himself as the strange man who drifted from Robben Island to Cape Town last year attached to a bunch of helium balloons.

But a team of scientists investigating his latest route have warned that he might face more than seagulls on his stroll across the seabed, about 8km as the crab crawls.

Not only must he navigate swirling ocean currents and unusual rock formations, he must avoid some deep-water potholes that could throw him off course.

That's according to a detailed high-definition survey by the SA Council for Geoscience, which agreed to help Silver-Vallance without charge.

It's the first detailed mapping of the seabed adjacent to Robben Island. It's also the first time anybody has tried to chart an undersea route back to the mainland.

"There are some things you have to worry about," said project leader Michael Machutchon.

"He can't just walk wherever he wants."

Initially the route looked easier than expected, thanks to an unexpected shallow underwater sand bridge linking the island to Blouberg beach.

About 15m deep, the sand bridge dips into a deep hole at one point, meaning Silver-Vallance will have to watch his step.

But the real surprise was the ancient rock crossing his path close to the entrance to Robben Island harbour.

The rock appears to be Malmesbury shale, one of the oldest rock types on the planet. It also underpins Table Mountain.

Fortunately, Silver-Vallance has political history on his side.

The "Freedom Walk" is a charity fundraiser for the Nelson Mandela's Children's Hospital, whose patron survived the most celebrated walk to freedom in history.

"It's all about validating Nelson Mandela's famous quote 'It always seems impossible until it is done'."

The hospital is under construction in Johannesburg. It is scheduled to open in 2016.

"The whole purpose of what we do is to look for projects that seem impossible," said Silver-Vallance, who is a business manager when he is not out doing the impossible.

One unexpected bonus of the route is an unusual reef built around large mussel outcrops that have attracted an abundance of sea life.

"We did a dive there and found really huge starfish and pipe fish.

"We didn't expect that," Silver-Vallance said.

Why is he doing this?

"I am a creative humanitarian. For me it is all about creating new ideas."