– In the extreme northeast corner of Iowa, on a grassy hillside ringed by meadows, limestone outcroppings and prismatic trout streams, an underground bunker safeguards our nation's food heritage.

Inside the surprisingly small 10-by-15-foot freezer vault at Seed Savers Exchange, floor-to-ceiling metal shelves are packed tight with white cardboard trays full of moisture-proof, foil-lined packets. The packets, labeled with long sequences of numbers and letters, hold the seeds of more than 25,000 varieties of old-time vegetables and plants.

But Seed Savers Exchange is more than just a repository. The nonprofit organization is the largest seed bank in the nation that makes its seeds available to the public, with the goal of reintroducing these nearly lost foods to back yard gardens, commercial farms and ultimately the American diet.

"We are the anti-Monsanto," executive director John Torgrimson says. "We are the safety valve. Before World War II, every farmer saved seeds. Today, patented seeds and hybrids make it impossible for farmers to save seeds."

Torgrimson says Seed Savers Exchange is also the backbone to the heirloom seed movement. "There's a good chance that a restaurant in New York is able to offer heirloom tomatoes on its menu because we've been doing this work for 34 years."

It all started with the seeds of two plants from Bavaria.

Diane Ott Whealy grew up on a farm near Festina, Iowa. Her paternal grandparents also had a farm nearby.

Shortly before her grandfather died in 1974, he entrusted Whealy and her husband, Kent, with seeds for two beloved plants that he had always grown: a large pink tomato and a red-throated purple morning glory.

Her grandfather's father had brought the seeds for both plants to the United States when he emigrated from Dreuschendorf, Germany.

The Whealys realized they were the last people in the family to have the seeds, and that got them thinking about the loss of genetic diversity in food crops nationwide. They wrote letters to Mother Earth News and other back-to-the-land magazines to try to find other people who were also saving heirloom seeds.

In 1975, the couple started True Seed Exchange out of their remote homestead in Princeton, Mo., 115 miles northeast of Kansas City, Mo. That first year, they printed a directory of gardeners who had seeds to share and sold it to 29 people who sent in 25 cents and a large envelope.

True Seed Exchange became Seed Savers Exchange in 1979 and moved to Decorah in 1986. Today Heritage Farm, as the headquarters is known, employs 50 people who work in the organization's research lab, trial gardens, greenhouses, visitors center and retail seed operation and generates $5 million per year.

"We started doing this before heirlooms were fashionable," she said. "We knew in our hearts it was the right thing to do."

Ott Whealy says it took a long time for the public to realize it needs the seeds her organization has worked so hard to find and distribute, but that only sweetens the gratification she feels now.

Kent Whealy left Seed Savers Exchange shortly after the couple divorced in 2004, but Diane Ott Whealy remains vice president and spiritual center of the organization.

Seed Savers Exchange is best known for its online and mail order catalog, which offers 600 varieties of heirloom vegetables and plants.

As charming and profitable as the catalog is (seed sales bring in about 70 percent of the organization's annual revenues), the heart and soul of the organization, the "exchange" part of Seed Savers Exchange, is the members-only yearbook, a listing that allows seed savers to connect with each other to trade, give away or buy and sell seeds.

Currently the organization counts 13,000 members in all 50 states and 40 countries. Membership costs $40 per year and includes a copy of the yearbook and other publications relating to gardening and seed saving. The 2013 yearbook lists 12,495 different varieties of heirloom vegetables and plants offered by 694 member growers. It includes 4,749 varieties of tomatoes, 875 types of peppers and 1,553 beans. The organization also provides monthly webinars at seedsavers.org that teach techniques for gathering and storing seeds.

"We did not just save the seeds," Ott Whealy said. "We gave people back the knowledge about how to save the seeds that had been lost."