In preparation for future Mars missions, the Hawaii Space Exploration Analog Simulation (HI-SEAS) is conducting experiments with researchers on a Hawaiian volcano. Six researchers returned from the Mars-like habitat on Friday.

The goal of the HI-SEAS mission is to simulate long duration Mars missions on Earth. The habitat is located on the northern slope of Mauna Loa - a volcano in Hawaii that is considered the world's largest - approximately 8,200 feet above sea level in an abandoned quarry. The dome that was built on the volcano has a laboratory, bathroom, kitchen, dining room, common workplace area, storage room, and exercise area.

On Friday six researchers returned from participating in a four-month HI-SEAS mission. Expedition leader Casey Stedman and five others spent 120 days during the second HI-SEAS mission consuming dehydrated food and only leaving the dome to conduct spacewalks in mock spacesuits. The researchers said communication with the outside world had a 20-minute delay each way to make the simulated Mars mission seem as realistic as possible.

These three men and three women were chosen to participate in the HI-SEAS mission out of 700 applicants that NASA sorted through. They were essentially experimental laboratory animals, as Kimberly Binsted, the principal investigator of the project, told the Huffington Post back in March that the participants would be undergoing a number of psych tests to examine their moods, interactions with one another, cognitive skills, and how it all changed over time.

The members of the mission were limited to eight minutes of shower time per week and were only allowed outside if they were conducting a 'spacewalk' with a simulated spacesuit that didn't give them much exposure to the outdoors. They were also subjected to work on a number of projects, including the testing of 3-D printed surgical tools, growing plants in harsh conditions, and recycling trash for other uses. The conclusion of the mission had the participants appreciative of the outside world.

"It's really great to taste fresh fruit and vegetables again," said Stedman, in this ABC News article. "The first thing I wanted to do after I come outside was to squint. We had little direct contact to the sun in the past four months."

You can read more about the HI-SEAS mission on its official website.