And article from 1930 that is still amazing today:

Secrets of the Mystery Gun that Shelled Paris

The secrets of the Paris Gun! For the first time in any magazine, Modern Mechanics here reveals the inside facts concerning the most startling and closely-guarded mystery of the World war—the official story of the giant German guns which, in 1918, dropped shells on Paris from a distance of 75 miles, a feat so incredible that artillery experts refused to believe it possible, thinking for a time that the shells were bombs dropped by high-flying aircraft. After the war the guns were destroyed and all information concerning them locked in secret archives. It was declared high treason, punishable by death, for anyone who possessed vital information concerning the guns ever to divulge it. Nevertheless, Col. Miller, author of this article and of the gripping book, “The Paris Gun,” obtained military pictures and technical secrets from confidential German sources which has enabled him to reveal to Modern Mechanics’ readers the astonishing story of the longest range guns the world has ever known.

ON THE morning of Saturday, March 23, 1918, as all the world knows, a supposed air bomb dropped into a Paris street and, when the fragments had cooled enough to be picked up} it was discovered they were marked with the lands and grooves of rifling, something no air bomb ever possessed.

By the time five or six more had crashed down, at intervals averaging about 15 minutes, the French artillery department had definitely decided Germany had accomplished the impossible and was shelling Paris with a gun that must be at least seventy, and probably seventy-five miles

In order for the gun to be accurate, they had to account for the curvature of the earth as well as the rotation of the earth. Each shot took 431 pounds of gunpowder to fire a 264 pound shell.

As described in this video, these huge artillery pieces did work, but they had a huge cost:

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