More than 550 people have gone to space, and only eight have reached Challenger Deep, the deepest point in the ocean.
But apparently, Kathy Sullivan can do both! Kathy Sullivan is the first American woman to complete a spacewalk. And now she descends 35,000 feet to the ocean floor, which is the deepest point in Earth's ocean that has ever been visited by a woman.
Before their expedition began, EYOS called on three daring explorers, also known as "mission specialists", to visit the bottom of the trench.
The former NASA astronaut Kathy Sullivan has successfully reached the bottom of the Challenger Deep. Its depth is approximately 6.9 miles (11,000 meters) below the surface of the Pacific Ocean.
Challenger Deep is considered to be the deepest point in Earth's oceans. It resides within the Mariana Trench, a gigantic, crescent-shaped depression that lies about 1,100 miles east of the Philippines.
In fact, it's so deep that the pressure at the bottom is more than 1,000 times sea-level pressure.
During the journey, Sullivan was accompanied by entrepreneur and deep-sea explorer, Victor Vescovo. They embarked on their journey aboard the DSV Limiting Factor, a deep-sea submersible, and their trip lasted just short of four hours.
This first-of-its-kind dive was part of the Ring of Fire expedition, which was organized by Caladan Oceanic, a sea exploration company founded by Vescovo.
Sullivan, however, is the first of the three explorers to finish the nearly 10-hour long trip.
"I know (Challenger Deep) as a bathymetric feature on a chart, a tectonic feature, and a seismic feature ... but that's all data-based understanding. To see it in person -- it makes all the difference in the world," Sullivan tells CNN Travel. "No self-respecting marine biologist would be able to pass up an invitation!"
Vescovo and Caladan were there to see the Five Deeps Expedition, which was an exploration of the 5 deepest points on Earth. This one, however, is supposed to be the first expedition of Challenger Deep to be documented in 4K video.
As soon as they returned from their expedition, EYOS organized a call between the pair of explorers and another pair of explorers who are believed to have made history: the two US astronauts who traveled to the International Space Station on SpaceX's Crew Dragon.
"As a hybrid oceanographer and astronaut this was an extraordinary day, a once in a lifetime day, seeing the moonscape of the Challenger Deep and then comparing notes with my colleagues on the ISS about our remarkable reusable inner-space outer-spacecraft," she said in a statement.
The new mission was funded by Vescovo and he sent "big congratulations" to Sullivan in a tweet, "Just back up from Challenger Deep! My co-pilot was Dr. Kathy Sullivan - now the first woman to the bottom of the ocean and a former astronaut as well as NOAA Administrator! Big congratulations to her! This was my 3rd time to the bottom. Well done by the crew, Triton, and EYOS."
Up until the dives began, the three explorers were fully briefed on the mission, as well as the schedule and research initiatives.
However, when it came to physical training, Rob McCallum, the co-founder of EYOS Expeditions and the Ring of Fire expedition leader, says it isn't exactly like going to space or climbing a mountain.
"These people are all adventurous, but you don't have to be an athlete to participate," McCallum told CNN Travel. "This is something new, but not something to be feared."
Kathy Sullivan took part in one of NASA's historic missions, STS-41-G. This mission, the sixth flight of the Challenger space shuttle, is known for being the first one with two women aboard.
And during the mission, Sullivan even performed a spacewalk for three hours and 29 minutes, which was the first that was ever performed by an American woman.
Sullivan's passion for exploration and adventure, however, has been with her since her childhood.
"I was always following the early astronauts, Jacques Cousteau, and the early aquanauts. They were inquisitive people. They were clever people that could figure out how to go make things happen," she recalls.
"That inquisitiveness, that sense of adventure, of curiosity that drives explorers. I could feel that resonating in me as I watched them."