A British man is trying to achieve what no swimmer has ever accomplished.  He is trying to complete what is called the "Ocean's Seven."

It features seven open ocean swims across some of the most challenging channels on Earth.

33-year-old Adam Walker has already swam across the English Channel and the Strait of Gibraltar -- two of the seven challenges.

Next up is a swim across the Kaiwi Channel from Molokai to Oahu.

The 26-mile crossing is considered the third most difficult swim of the "Ocean's Seven."

"It will push me to the limit," said Walker.  "I've never swam over 11-and-a-half hours before.  I think this might be 14 to 15 hours depending on tides and weather."

Walker's Kaiwi Channel crossing is being sponsored by Equus Hotel in Waikiki and the makers of the Electronic Shark Defense System.

Walker is also raising funds for two charities:  SportsAid and the Whale and Dolphin Conservation Society.