Reinhardt Teschke, 54, from Germany, was part of a three-man team which dived yesterday on the wreck of the Minnehaha, which lies in about 90m of water, about 20km south of Cape Clear.
Only his two dive companions returned to the charter boat Holly Jo. The boat’s skipper, Colin Barnes, who operates out of Reen Pier in Castletownsend, said he raised the alarm at about 5.30pm once it became clear that Mr Teschke was overdue.
“They were diving in glass-calm conditions,” Mr Barnes said.
“Reinhardt is a good and very experienced diver and had all the proper gear.
“The visibility over the wreck site was good, and it was a fine, normal dive for his companions. But it’s a complete mystery what happened to him.”
Valentia Coast Guard launched a major search operation involving two RNLI vessels from Baltimore and one from Courtmacsherry, the Shannon-based Coast Guard helicopter Rescue 117, and naval vessel LÉ Ciara.
Mr Barnes held his catamaran over the dive site as a reference point for the searchers.
The search was called off as darkness fell and was due to resume at first light today.
The defensively-armed British merchant ship was built in 1900 by Harland & Wolff in Belfast.
She was 20km south-east of the Fastnet on Sept 7, 1917, when she was torpedoed by a German U-boat and sunk with the loss of 43 lives.
Today, the wreck lies in about 90m of water, a depth considered far beyond recreational diving.
Diving at the site is considered to be one of the most challenging, complex, and risky dives in Irish waters.
Mr Teschke and his two friends came to West Cork specially to dive this wreck.
Dive experts describe it is a “technical dive” which should only be undertaken by the most experienced of divers.
Unlike recreational dives when people operate in “buddy pairs”, technical divers often operate alone over wreck sites.
And because of the depth, they have to use re-breathers containing a special mix of helium.
They can only operate at that depth for about 30 minutes and it takes up to three hours to resurface safely to avoid experiencing the bends.
As well as the hazards of diving to such depths, parts of the wreck of the Minnehaha have become wrapped by nets, which can be lethal if divers get caught.
There are no dive rescue teams in Ireland licensed to operate at that depth.