Without trees, trenches, or other terrain to provide cover, the open ocean made for an especially dangerous battlefield during the 20th century for aircraft and watercraft alike. Ships were sitting ducks that you could spot from miles away, and those ships could easily see—and target—approaching planes. So militaries developed smoke screens to help conceal their forces.

According to PortandTerminal.com, an early version of the smoke screen entailed funneling hundreds of gallons of oil through a ship’s combustion chamber, generating massive plumes of smoke that flowed from the smokestacks. This type of smoke screen was used occasionally during World War I, but it took too much oil—and the screen vanished too quickly—for it to really catch on.

Toward the end of the war, militaries had landed on a more promising alternative: titanium tetrachloride. While the yellow liquid could corrode metal and seriously irritate your eyes and lungs, it could also create an impressive curtain of dense white smoke that looks a little like the colossal northern Wall from Game of Thrones.

As Popular Mechanics reports, titanium tetrachloride is probably responsible for the smoke screen seen in the footage below. Filmed by the U.S. Army Engineering Division in 1923, the video displays a U.S. biplane spewing smoke near a battleship with the caption “To protect the bombing plane the Air Service can lay a smoke curtain.” This particular smoke screen didn’t occur during battle—it was part of a trial series Army Brigadier General William Mitchell spearheaded to prove the Air Force could compete with the power of the Navy. The ship was likely either the USS Virginia or the USS New Jersey.

Though smoke screens were a clever innovation at the time, technological advancements have since made them unnecessary and ineffective. Radar and infrared devices aren’t fooled by smoke, and they also allow the Air Force to fire missiles from afar. And if you do need to conceal the whereabouts of your craft, you can interfere with your enemy’s radar.

[h/t Popular Mechanics]