(CNN) -- OK, so it looks like something from a horror movie.
But a customized silicone mask, fitted over the face, head and mouth of endurance swimmer Diana Nyad, was a key difference-maker in helping the 64-year-old complete the grueling, 100-mile swim from Cuba to Key West, Florida, on her fifth try.
Nyad needed the mask for protection against box jellyfish, deadly creatures common to tropical waters and whose venomous sting cut short her fourth attempt to swim from Cuba last summer. Because the mask is cumbersome, she only wore it Saturday night when her support team feared jellyfish were nearby.
"I was grateful for it. I knew I wasn't going to be stung at all. I felt 100 percent prepared for the jellyfish," Nyad told reporters Tuesday morning after her record swim.
But the design of the mask, with its narrow mouth opening, was not perfect. For 13 hours Saturday night and early Sunday, it caused her to swallow "tremendous volumes" of seawater, which in turn led to bouts of vomiting. Nyad grew dehydrated, weakened and discouraged.
"That night was hell on Earth, it really was," she said.
The jellyfish dispersed by Sunday afternoon, however, and Nyad did not wear the mask again during the crossing.
Venom from box jellyfish contains paralyzing toxins that attack the skin, heart and nervous system and is considered to be among the most deadly in the world. Twenty to 40 people die from stings from box jellyfish annually in the Philippines alone, according to the National Science Foundation.
During last year's swim, Nyad wore protective gear over her face, hands and body. But the jellyfish still stung her repeatedly in the mouth, aborting her attempt about halfway between Cuba and Key West.
"Literally the only square inch exposed of my entire body was the lips. We just couldn't design a way to protect the mouth and still breathe while swimming," she said last month in a blog post on her website. "Yet these animals ... are brilliant at finding animals to sting and they indeed found my lips. On both occasions, I suffered the paralysis, the otherworldly sensation of being burned alive."
So Nyad partnered with Stefan Knauss, a California prosthetics expert who spent a year developing the silicone mask. They tried many molds and different designs of the mouth area before finding one that worked best. Nyad tested the mask, along with her other protective gear, by swimming through a swarm of hundreds of box jellyfish in June.
"As difficult as the swimming was, I was not stung once," she said. "Those deadly tentacles could not penetrate."
And indeed, the jellyfish were not a problem on Nyad's triumphant fifth crossing, which she completed Monday afternoon after nearly 53 hours in the water.
But the jokesters of Twitter had some fun with her mask.
"Not only did Diana Nyad finish her swim, but that new mask of hers can double as a Halloween costume. One-stop shopping. Smart lady," wrote a Boston-based blogger on Twitter.
And there was this, from a Kansas-based comedian: "Somewhere under the sea the King of the Jellyfish yells at his men "WE HAVE FAILED. DIANA NYAD'S SILICONE MASK HAS SHAMED US ALL."