WASHINGTON — Four astronauts resurfaced July 29 from an undersea laboratory where they had spent the past nine days honing space exploration skills while learning to live in close quarters. The expedition marked the first time in two years that NASA has sent astronauts to the Aquarius Reef Base research station, a pressurized habitat 19 meters beneath the Atlantic Ocean near Key Largo, Florida.

Since 2001, NASA’s Extreme Environment Mission Operations (NEEMO) project had typically leased the undersea facility one to three times a year to give astronauts, engineers and scientists the chance to conduct experiments, test technology and simulate spacewalks. But a planned 2013 expedition was scrubbed when the University of North Carolina at Wilmington, which had been running Aquarius for the National Oceanic and Atmospheric Administration since the six-person habitat was relocated to the Florida Keys from the U.S. Virgin Islands in 1992, gave up on trying to keep the facility financially afloat. Miami-based Florida International University stepped in early last year as the new caretaker, suspending operations while it worked to reopen Aquarius with a mix of public and private funding.

The first of two NEEMO expeditions planned for this year began July 21 when an international crew commanded by Japanese astronaut Akihiko Hoshide entered Aquarius for the first time. He was joined by NASA astronauts Jeanette Epps and Mark Vande Hei, European Space Agency astronaut Thomas Pesquet, and habitat technicians Hank Stark and James Talacek. The crew conducted simulated spacewalks to test out new drills, cutting tools and techniques for attaching people to surface points — procedures NASA says will be crucial for human expeditions to asteroids.

The primary focus of the expedition, dubbed NEEMO 18, was human health and habitability. The astronauts conducted human behavior experiments using health badges that monitored their heart rates, hormone levels and exposure to light while also tracking the astronauts’ proximity to each other. The badges were designed to help understand how people interact with each other in close environments.

NEEMO 18 also subjected the crew to the kinds of communications delays astronauts would experience communicating with ground control during missions to asteroids and other deep-space destinations.

NASA plans to launch its second NEEMO expedition of 2014 in early September. That expedition, dubbed NEEMO 19, will focus on evaluating so-called telementoring operations where undersea crew members are coached by land-based experts via video and voice communications.

SpaceNews editorial intern Sydney Mineer is studying journalism at American University in Washington.