In 1869 a giant space gun was built, with the intent of firing a projectile at the Moon, just to prove that they can do it. Of course, the ludicrous idea to put people inside it was never even taken into consideration, as the acceleration would kill them, and if not, the crash into the Moon, or the lack of an atmosphere would finish with them : it just fired a normal cannon shell.
The experiment was a success, the cannonball hit the Moon and formed a nice little crater, an event being seen and photographed by astronomers. However, as it didn't have any other use, it was quickly abandoned after the first experiment. They coated the interior with wax and put a cover on top of it to keep rust and rainwater away, so it survived mostly intact to his day.
Could we find any useful propose for it? Could it have been of any use in the space race of the 1960's and 70's?
I guess the only benefit is that it's already built by our ancestors so it is completely free: we don't have to spend money and resources for building it. However, as it is built completely vertically, it's not useful to achieve (unaided) orbit around the Earth. Also, the high initial acceleration would probably destroy any sensitive electronics or other equipment we tried to launch with it. Could we design projectiles to limit this? If we can't find any use for it now, could we in the not too distant future?
The only use I might see in it, is to send bulk raw materials to a space station or a Moon colony, but we have neither a Moon colony, nor a space station requiring bulk raw materials.
The cannon is as close to Verne's idea as possible, has a bore with a diameter of nine feet (or slightly less, but not by much), is cast directly and completely into the ground on a hilltop in Florida, is completely vertical, and long enough to be able to be used to hit the Moon with a single explosive charge available in the late 19th century. If the later proves to be physically impossible, then we can modify the original experiment so that the cannonball left the atmosphere, but failed to reach the Moon, instead falling back and splashing into the ocean.