German-Era Bomb Shelter Signs In LvivDuring WWII, Lviv was occupied by Nazi Germany from 1941 to 1944. The Germans established bomb shelters around the city and marked them with "NA" and an arrow pointing downwards. Some of these signs remain today. "NA" probably stands for "Nachrichten-Abteilung," which means "intelligence unit."
These have long gone unused, but, unfortunately, due to a new war, today on the other side of the country there are active bomb shelters.
Here are some of the signs.
Most of the examples I found are on buildings built in the interwar period. Maybe these were chosen specifically because they were newer buildings and therefore probably structurally more sound than the older Austrian-era buildings.