An unusual scientific experiment ended 25 years ago when eight people walked out of a miniature Earth called Biosphere 2.

Biosphere 2 is a collection of large, connected buildings in the American state of Arizona. The buildings are made mainly of steel and glass.

The experiment began in September 1991 when the eight individuals entered Biosphere 2. A crowd watched as the four men and four women were secured inside. The event received a lot of media attention in the United States and across the world.

Biosphere 2 was designed to test whether human beings could survive in a closed environment, one that aimed to create conditions similar to those found in nature. The miniature Earth had a man-made rain forest, desert, grasslands and even a small ocean.

In this July 31, 2015 photo, tourists check out the Biosphere 2 Ocean, which holds a million gallons of seawater, in Oracle, Ariz. (AP Photo/Ross D. Franklin)

Technology provided air flow, water and temperature controls. Water supplies and air were cleaned and reused. The eight people grew vegetables, rice and other foods they needed to survive. Farm animals were used to provide meat, eggs and milk.

The eight test subjects, called "Biospherians," faced a few unexpected problems during the experiment. Low oxygen levels were discovered and began to affect the group's health. A decision was made to pump in additional oxygen. The group was also not able to produce as much food as expected.

But all of them were able to complete the survivability test, which ended in September 1993. By then, the Biospherians had divided into two opposing groups. They had split under the pressures of sharing a small space and disagreements over the direction of the project.

The decision to provide oxygen from the outside world led to criticism of the $150 million project. Some members of the scientific community called the experiment a failure because the closed environment was supposed to be self-sustaining.

In 1994, another experiment was launched with a new group of subjects after improvements were made to Biosphere 2. But it had to be stopped six months later because of disputes over administration and financing issues. The ownership of Biosphere 2 then changed several times over the years.

Today, the University of Arizona operates the buildings as a scientific research center for controlled studies. Researchers from around the world can now use the facility to carry out experiments.

Joaquin Ruiz is a geologist with the University of Arizona and director of Biosphere 2. He told the Associated Press the controlled settings can help researchers to complete experiments in a more predictable environment.

Among the users of Biosphere 2 have been researchers from the Global Institute for Water Security at Canada's University of Saskatchewan. They studied how plants in the small rainforest deal with water intake without risking harm to the environment.

Scientists from the University of Hawaii have carried out experiments on a miniature coral reef without hurting real corals in the Pacific Ocean.

Christopher Field is the director of the Stanford Woods Institute for the Environment. He told the AP he thinks Biosphere 2 has developed into a valuable, modern scientific tool.

"You have to separate it from what it was originally to see its worth today," Field said. He added that Biosphere 2's controlled environment provides "a powerful way to help us understand the way the world works."

Jane Poynter was a member of the first Biosphere 2 group. She told the AP she thinks the current operators of the project "have been doing a really great job" at developing "strong foundations" for science. She added: "Twenty-five years after we came out, it's still very forward-looking."

The Associated Press reported this story. Bryan Lynn adapted the story for VOA Learning English. George Grow was the editor.

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