The Hyattsville Post Office was once a fallout shelter.

A small sign near the door of the post office on Gallatin Street identifies it as part of the Community Fallout Shelter Program, a federal program which began in September of 1961. It’s a historical marker you’ve probably walked right past.

Back in those Cold War days, people were concerned about a nuclear attack from the Soviet Union. Aside from the initial blast, they worried about ways to avoid “The Day After” effects of radiation poisoning.

The post office’s sign identifies it as being capable of holding 70 people, so the building at one point would likely have stored sanitation kits, barrels of water, cases of food and radiation detection kits for that many.

Federal funding for the program ended in the mid 1960s, but some shelters remained stocked through the 1970s and 1980s, according to one history.

Today, it is the markers themselves that are at risk. A study by D.C. residents Adam Irish and Melissa Hopper, founders of the informative District Fallout website, found that just 5 to 10 percent of the signs in the District were still intact.