Alright, I've spent too much time looking for the fucking screencap. Going off of memory.
Dig a big-ass hole in the ground. Build it to be as perfectly spherical(?) as possible, line it with concrete etc. Fill it with water, suspend a nuclear explosive in the middle. Out the top of the extremely reinforced hole, you have a pipe which is capped with a small disc of copper.
On detonating the explosive, because the size of the hole has been calculated to be just enough to hold an amount of water specific to the yield of the explosive, the entire quantity of water vaporizes almost instantly. Thousands of gallons of water, instantly flashed to steam and looking for a way out. The ground isn't moving, so it takes the only route available: the pipe, which is "hopefully" pointed at your target.
Something something, the copper disc violently deforms and accelerates, experiencing so much force that it's speed hits double digits of a percentage of the speed of light.
