When I pour my coolant in, it circulates, and.

Most likely a core plug, also called a freeze plug. They're about 1 1/2" in diameter. All auto parts stores have them and they can look up the correct size for your engine. They may also have a tool to borrow that makes installation easier. They often corrode through and leak. Sometimes one is removed and an engine heater is installed in its place. You can also get rubber plugs that expand when you tighten a center bolt.