Since the earliest days of space training, NASA has submerged astronauts underwater to simulate the weightless experience of moving in microgravity. In 1966, for example, Buzz Aldrin practiced spacewalks with a mockup of a Gemini spacecraft in a large pool in Maryland to prepare for the Gemini 12 mission. In 1983, astronauts practiced moving around a full-scale model of the space shuttle’s cargo bay using the Johnson Space Center’s weightless environment training facility (WETF). The success of underwater training eventually prompted the creation of the Neutral Buoyancy Laboratory in Houston, the largest indoor body of water in the world , capable of holding models of major sections of the International Space Station.

Beginning in 2001, astronauts have been training and testing equipment in the open ocean as part of the NASA Extreme Environment Operation, or NEEMO, program. Last month, a crew of aquanauts and astronauts surfaced after spending nine days in an undersea habitat known as the Aquarius Reef Base , located 62 feet below the ocean surface off the coast of Key Largo, Florida. The expedition, NEEMO 23 , was the latest in a series of missions that use the floor of the Atlantic Ocean as a testing ground for possible deep space technologies and procedures. At only about 40 feet long and 20 feet wide, Aquarius is also an ideal testbed for studying the physical and mental effects of living and working in confined spaces.

Though the program has been around for nearly two decades, this was the first NEEMO mission to have an all-female NASA research team. NEEMO crew member Csilla Ari D’Agostino, a neurobiologist at the University of Southern Florida, explains that the data gathered over the course of this mission is key to creating a complete picture of how all astronauts might behave in space, especially since past NEEMO crews were primarily male.

“It’s important to collect data on both genders,” Ari D’Agostino says. “If there’s a difference we need to figure out how to manage that. If there’s not then we need to know that, too.”

Ari D’Agostino was joined by aquanaut Shirley Pomponi, a marine biologist at Harbor Branch Oceanographic Institute of Florida Atlantic University, and NASA astronaut candidate Jessica Watkins. The team was led by Italian astronaut Samantha Cristoforetti, who spent 200 days in space in 2014 and 2015—a record spaceflight for a European.

The non-astronauts of the group were chosen for their diving experience and to conduct their own research. Ari D’Agostino studies the physiological effects of diving, such as what happens when divers breathe oxygen in high pressure environments. During the expedition, she also studied group dynamics and how behavior changes under physical and mental stress. Pomponi, on the other hand, studies sea sponges and tested a new handheld sampling tool that could have potential applications in space, such as sampling planetary rocks and sediments.

While previous NEEMO missions focused on simulating activities on the surface of an asteroid , NEEMO 23 tested new equipment and working protocols for the International Space Station and future deep space missions to the moon and Mars.

On a typical day, half of the crew members spent up to five hours outside the vessel testing equipment and collecting samples, while those indoors were in charge of directing the work and communicating with mission control. “Sometimes I’d be communicating with four different people at one time,” Pomponi says. “I’d be communicating with the divers, with science communications, with mission control and with somebody inside the habitat.”

Inside Aquarius, Pomponi instructed the divers outside on how to use her research equipment for collecting sponge samples and measuring their metabolic activity. This method served as an analogue for future missions to Mars, where an astronaut might be tasked with using specialized tools designed by someone else.

“There was a little bit of training on land beforehand, but not much,” Pomponi says. “But in space, when they’re exploring, the astronauts who are up there are not necessarily the ones who know how to use those instruments.”

Sixty-two feet below the surface, the team also ran into logistical problems: Devices struggled to connect to the internet, microphones cut out and helmet cords came loose, among other mishaps. For each of these scenarios, the team had to find a fix for the issue on their own. Pomponi says her experience communicating with divers to operate and troubleshoot her equipment illustrated the importance of having someone with scientific expertise as part of the crew, rather than just at mission control.

“[If] you’re on Mars, and you send a message back to mission control, it’s going to take 20 minutes to get [there], and then another 20 minutes to get the response back,” Pomponi says. “So at minimum, you’ve got a 40 minute delay. This mission was specifically designed to think about giving astronauts the opportunity to troubleshoot and … to make decisions on their own that were related to the procedures or instruments that we were operating.”

Team members inside the habitat also evaluated tools such as a scanning electron microscope (SEM) that is bound for the International Space Station this year. “It’s like a miniature version of an SEM,” Ari D’Agostino says. “It’s absolutely amazing because normally you would need almost half of a room, and this was like the size of a coffee maker.” The team used the SEM to image samples like Pomponi’s sponges, and since the pressure inside Aquarius is 2.5 times higher than that at the surface , the aquanauts and astronauts could also report how the microscope operated in an extreme environment.

Outside of Aquarius on the seafloor, team members tried out a number of new devices for possible use in space. They tested a new piece of medical equipment, a moon stretcher called the Lunar Evacuation System Assembly (LESA) that helps lift and transport an incapacitated astronaut—a daunting task without mechanical assistance, considering the aquatic spacesuits include a 32-pound helmet, an umbilical line and a 10- to 20-pound weight to prevent the aquanauts from floating to the surface. In space, an astronaut’s equally bulky outfit and the lack of gravity would make this task just as difficult, highlighting the advantages of testing these new designs underwater before sending them to another planet.

To assess how the living conditions impacted their mental aptitude, the crew performed daily exercises such as a lunar flight simulation, which tested their ability to multitask by requiring them to simultaneously land the vehicle, read incoming communications and call out metrics like altitude.

Part of Ari D’Agostino’s research involved studying how the group behaved as a whole. While some teams might crumble under mental and physical pressure, the NEEMO 23 crew collaborated seamlessly.

“Obviously there were very stressful moments when everyone was very task loaded, the habitat was very small and we couldn’t get out of each other’s way,” Ari D’Agostino says. “We really needed to know when somebody needs help, when somebody needs to step in and when somebody needs to step back. It was a very, very good mix of personalities, and we had a lot of fun.”

Now back above sea level, the team is dispersed and will spend the upcoming weeks analyzing their data and eventually reporting the results. Ari D’Agostino hopes she can contribute to future NEEMO missions as part of the support crew.

“We were laughing a bit with astronaut friends that we had postpartum depression now that this big thing is over, and we were just trying to reflect,” she says. “I think we needed a little time to digest what really happened, because something could have gone wrong and luckily didn’t. We experienced some really exciting and big things.”