Subspace Munitions Range 434 was a region of space near the border of the Delta and Beta Quadrants that was utilized by an unknown species as a testing grounds for their long-range subspace warheads.
The USS Voyager stumbled upon the region in early 2378, where they discovered it to contain high levels of subspace radiation. Due to the high levels of subspace radiation, caused by warhead explosions, Voyager was unable to create a stable warp field, forcing it to travel at impulse for nearly two days.
Along with radiation, a further evaluation of the region revealed large amounts of metallic debris, which suggested that dozens of warheads had been fired over the past several weeks. (VOY: "Human Error")