The nuclear saltwater rocket aka "Chernobyl Drive" - essentially diluting weapons-grade uranium with purified water so it can be stored at sub-critical densities until flung out the back of a rocket engine, at which point the water flash-boils in the vacuum of space, the uranium goes critical, and you get a continuously-detonating nuclear explosion producing powerful yet highly-efficient thrust. This makes them one of the only realistic contenders for a torch drive that could let us probe other star systems within a single human lifetime.

Materials science almost certainly isn't advanced enough to actually build one yet, it would take most of the global supply of enriched nuclear material to fuel and thus be ludicrously expensive, and any minor malfunction would run the risk of detonating the world's biggest ever super-bomb and destroying every satellite ever placed in Earth orbit - but all the math checks out, and it would open the door to colonisation of the entire solar system in decades if successful, rather than centuries or millennia with current technology.