Historic 103-mile swim aided by electric shark shield

Some 24 hours into her journey from Cuba to Florida, distance swimmer Diana Nyad is using an electronic shark repellent in her attempt to break the world record for swimming the longest distance without a shark cage. The record is so tough it has been held for 32 years--by her, when she swam 102.5 miles from the Bahamas to Key Florida in 1979.

Everything about the swim is impressive. For starters, Nyad is 61. And then there's the fact that she didn't swim a single stroke for 31 of the past 32 years. Now she is trying to swim 103 miles without stopping. That's at least 60 hours of nonstop swimming--and she's doing it in the open ocean, battling big waves, fatigue, her own physical limitations, and, of course, shark-infested waters.

While Nyad has made it clear that she knows she is in shark territory and does not want to hurt them, sharks are able to pick up her heart beat, so she has chosen to swim using a pulsing Shark Shield.

The Australian-based company's general manager, Martin Grace, says you can actually see the shark's skin shudder when it reaches the shield--which works much like an electric fence for dogs--and that ideally the shark will simply have a look around and eventually swim away.

Its shield helped swimmer Penny Palfrey set the world's unassisted cage-free long-distance record this past June, a 67-mile open-ocean swim between two Cayman islands. (The distance between Cuba and Florida was actually crossed in 1997, but with a shark cage and by a 22-year-old.)

Grace tells Discovery that the Shark Shield is nonlethal--and that it feels to the shark something like a big punch in the face. It produces electrical waves underwater that cover a 10- to 16-foot radius, and in Nyad's case is affixed to two nearby kayaks.

But Shark Shield doesn't deter all sharks. In 2008, a surfboard carrying a shield was not only not a deterrent to a shark, it was food. As in, the shark simply ate it. And even when it works as a deterrent, the shield is just that. Palfrey's 40-hour swim in June, for instance, left her with a series of issues to be treated in a nearby hospital: dehydration, blistering of the mouth, bruising, and swelling of the face, legs, trapezium muscles, and tongue.

Before setting out at dusk Sunday night, Nyad told CNN that she is also accompanied by not one but four shark specialists who are taking turns patrolling nearby activity and attempting to divert any oncoming sharks. She stressed that under no circumstances will her team kill a shark.

Given the wide range of dangers Nyad faces during this incredible challenge, not least of which is the asthma she has already had to battle during the first day of her swim, sharks may ultimately be the least of her concerns.

To follow Nyad's journey in real time, check out her Twitter feed or follow her blog, which members of her team will be updating 24/7 these next few days.

Update August 9 at 9:54 p.m. PT: After 29 hours, Nyad stopped her swim due to a combination of asthma, a shoulder injury, and excessive vomiting. While she may not have bested her record set in 1979 of 102.5 miles without a shark cage, the 61-year-old very well may have set a new record of 29 hours for longest swim in her age group. In Florida, she said: "I do not feel like a failure at all. But we needed a little more luck."