KAILUA-KONA, Hawaii — When the cannon boom echoes across Kailua Bay at 6:45 a.m. Saturday, it will mark the start of one of the world's most strenuous contests for 1,800 triathletes.

The Ironman World Championship, which celebrates its 30th anniversary this year, challenges athletes with a 2.4-mile swim, 112-mile bike ride and 26.2-mile run.

The route starts and finishes at the Big Island's Kailua Pier, but threads through miles of hot, windy, barren lava fields to the northern tip of the island and back again.

Ironman triathlons are held throughout the world but the Hawaii race is considered the most prestigious. Many say it is also the most challenging.

The first Ironman was held in Honolulu in 1978, when several Navy officers decided to plunge into the ocean for a Waikiki rough water swim, before bicycling around Oahu, and running the Honolulu marathon course — all in one day.

The race later moved to its current Big Island location.

It's a race and course that have been said to curse some athletes, including the defending men's champion, Chris McCormack.

The 35-year-old Australian was thwarted six times before he finally earned the title last year.

"When I crossed the line, it was a huge relief to have that monkey off my back, to finally put my name on that trophy and fulfill my destiny and my dreams and hopes for myself," he said. "Now I know I can win the race. I don't have to prove myself. I can just enjoy it more."

He will be challenged by last year's runner-up, Craig Alexander, also of Australia. Two-time champion Normann Stadler, who withdrew from last year's race, and 2005 champ Faris Al-Sultan will be competing.

The defending women's champion, Chrissie Wellington, aims to repeat her victory.

The 31-year-old former U.N. worker and native of Great Britain was competing in just her second Ironman distance race when she took the title last year. She's won five more competitions in the 12 months since.

"The crown can weigh heavy sometimes, as they say, but I'm happy that I've proved I'm not a one-hit wonder and I can handle the pressure," Wellington said.

Last year's runner-up, Samantha McGlone of Canada, will not take part after suffering an Achilles tendon injury.

This year, the athletes range in age from 18 to 78, from 50 countries and 46 states. The first place man and woman each receives $110,00, but the vast majority of athletes will take away only a coveted finishers' T-shirt, a medal and bragging rights that will last the rest of their lives.

During the race, nearly 5,000 volunteers will staff aid stations along the entire bike and run course, handing out 20,000 gallons of cola, water and soup, 10,000 energy bars, 1,350 bread rolls and 66 cases of bananas.