Before the invention of the the telephone or the two-way radio, call boxes helped firefighters and police in the District stay connected through a system of underground cables.

This video from Vox looks at the century and a half of the call boxes’ history, which dates back as early as the 1860’s.

The fire call boxes were installed first, employing a telegraph system so people could pull the key if they saw a fire nearby. The different call boxes were all connected to the central alarm center, which could tell which box was triggered and send along firetrucks accordingly. The video says you can identify the earliest ones by their “harp shape.”

Police boxes served a different purpose. Oval shaped, the boxes allowed officers to check in with headquarters before two-way radio came about.

While cities like San Francisco and New York City still have operational call boxes, all that remains in D.C. is the old-school cast iron frames—the city removed all of the working components by 1995.

With a project called “Art on Call,” Cultural Tourism DC worked from 2000-2009 to restore and decorate many of the frames with art.

But not all of the art featured on the call boxes is recommended. A passerby called the bomb squad on one embellished box on 15th and L streets NW in March 2016, and it was taken down. “As much as I am a fan of public art, maybe it wasn’t the greatest idea to put up wires in downtown D.C.,” Ryan Moorman, a product specialist at NGP Van who watched the investigation from his office, told DCist.

Here’s where you can find the decorated call boxes that no one has reported to the bomb squad (yet):