Becky Baxter has a vivid memory of her daughter’s first swim lessons. Sarah Thomas was the eager one-year-old girl in the toddler-filled class, which was aimed at helping kids not panic if they fell into a pool. While others cried, Thomas never hesitated, keeping her eyes on her mom even as she was pulled underwater.

The oldest of four girls, Thomas has always been the one to confront challenges without fear, according to Baxter. So, for those closest to her, it wasn’t a surprise when the now-37-year-old decided she wanted to tackle a feat that had never been done before: swim the English Channel four times, nonstop.

The idea came when Thomas was part of the crew for a friend’s swim across the English Channel. As she watched, she wondered what it would take to swim back and how far she could go. Thomas was convinced to try open water swimming when she was 25, after a move to Denver following a swimming career in high school and at the University of Connecticut. Her first 10K in 2007 later turned into 20 miles, and then 40 and 50 miles, which then led to even longer distances—and then record books. Swimming the Channel for the first time in 2012, she’s also completed a two-way crossing of Lake Tahoe, crossed the Catalina Channel and the Cook Strait in New Zealand and more.

So after swimming 104.6 miles in Lake Champlain—setting world records for the first current-neutral swim of over 100 miles and the longest unassisted open-water swim—Thomas decided to put her efforts towards a quad-crossing attempt.

At the top of her sport following Lake Champlain, Thomas had already placed a deposit on a boat (most open water swimmers opt for one as an escort system and safety method in the water) and she planned on focusing her training on her new goal. But in October 2017, a shocking discovery halted her plans: Thomas found a lump in her breast and shortly after, she was diagnosed with Stage 2 breast cancer.

Thomas suddenly faced the diagnosis and all that comes with it, including dreadful chemo treatments and unknown health scares. But as she battled the cancer, Thomas continued to swim as much as she could. During her fight, Thomas received an email from the boat captain she’d previously chosen to lead her Channel journey. Through Thomas’ Facebook posts, he’d been following along with her and wondered if she still planned on coming to England. Am I going to do this? Thomas thought to herself. With additional payments needed to be made on the boat, Thomas responded to the captain with something along the lines of, “Yep, tell me how to pay you.” She was in.

From there, Thomas made a year-long training plan. She was 20 pounds overweight from all of her treatments and wasn’t as fast as she used to be. Since the diagnosis, Thomas also had reconstruction on her right breast, which changed her muscle structure. She worried about having limited shoulder mobility and a lack of power in the water. She also worked a full-time job while training. But she says she wouldn’t have traveled to England if she didn’t believe in her ability to complete the swim. Before her attempt, Thomas dedicated her swim to “all the Survivors out there.”

“I never wanted to be limited [or] defined by my cancer journey,” Thomas says. “I think our lives are much more complex than that.”

In the early morning hours of Sept. 17, Thomas jumped into the water and set out to swim the four-way English Channel attempt. A grueling 54 hours later—after several bouts of nausea, vomiting, jellyfish stings and battles against the currents—Thomas officially became the first person ever to swim the English Channel four times non-stop, swimming from England to France and back two times.

“Coming back the way she did, so quickly, and being able to swim to this caliber, it's just a really strong statement about how she didn't lose her identity to cancer,” says Elaine Howley, Thomas’ friend who swam part of the journey with her. As many changes as it brought into her life, she's still her and she's still kicking ass and taking names.”

Before her swim, Thomas carefully selected a small pebble from her home reservoir in Colorado and stuffed it down her suit by her hip to keep it secure for the swim. A few times throughout, she reached for it as a reassuring reminder that everything was OK. When Thomas finally landed back on the English shore, she was greeted by a crowd bearing champagne and M&Ms. And she left the Colorado pebble on the beach, trading it for an English one as a memento of her feat.

Her husband, Ryan Willis, says swimming provided an outlet for her, giving her a purpose as she fought cancer. Thomas says her accomplishment shows that cancer doesn’t have to stop you.

“I think it's a good message to people who are in the middle of it to have a little bit of hope when it feels like you'll [never] move past it or not be sick again,” Thomas says. “It's a dark place in the middle of cancer treatment…[but] you can still do everything that you were doing and you can still have dreams and goals and make plans.”

In a Facebook post from July following a training session in about 62-degree water, Thomas recalls a fisherman telling her she was “bat s--- insane.” Fifty-four hours of no sleep, swimming 84 miles of the English Channel and puking in the dark while you’re swimming may seem that way to most people. But not to Thomas.

“To me, it's more like a compulsion,” she says. “I need to do this. I need to be in the water. I need to be finding limits and pushing boundaries, so it doesn't feel crazy to me. It just feels like something I have to do. I need to do.”