This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

(NewsNation Now) — Some 15,000 Ukrainians have been hunkering down in subway stations for protection since the start of the invasion, according to Kyiv’s mayor. As it turns out, that’s what these subway systems were designed for.

Currently, thousands of Ukrainians are hiding underground as air raid sirens can be heard throughout the city.

The capital city’s metro system was completed in the 1960s when Ukraine was part of the Soviet Union. It includes stations built deep underground, designed to double as bomb shelters in the event of an enemy attack.

Kyiv’s Arsenalna station is believed to be the deepest in the world, located 346 feet below ground. The majority of all subway stations in the city are underground and 20 are considered deep, with a depth of 65 feet.

The deep cover helps protect civilians from experiencing the brunt of blasts and any fires afterward.

The subway structure is the same in Kharkiv, where there are at least six deep stations that help double as bomb shelters.

Ukraine is not the only nation to make use of subway systems as blast shelters. London residents took shelter in their subway system during the German blitzkrieg in World War II.