The Atomic Submarine gave us battles under the ocean with a UFO.
An atomic submarine investigates and finds the cause of mysterious ship disappearances. Then they have an undersea battle against an invader that looks a lot like The Crawling Eye.
Visit toy departments and look for giant crab monsters, I know I do! A group of scientists are on an island and are attacked by giant crabs. The island falls away bit by bit and the crabs get more and more victims. I imagine an attack on a coastal city by giant armored crabs, nothing can stop them!
The planet is headed for destruction but people are building an ark to save themselves, but you have to fend off the rioting mobs!
Chinese Communists are tunneling under America for underground conquest. After a game of Them! us the giant ant nests as nests for Commies!