Moon Madness (or The Secret of the Selenites)

The film follows the adventures of Baron Munchausen, who is prompted to travel to the moon by his cousin Sirius, an astrologer convinced that it is inhabited by an ancient race called the Selenites.

The "within story fact" that they live on the Moon, and Earth's Moon is also known as "Selene" (as our Sun is also known as "Solaris") supports Sirius' theory.

The Selentites incidentally possess the secret of immortality. The Baron takes up his cousin's offer and travels to the moon using a tall masted ship pulled by three hot air balloons and he is aided by his super-ability friends.

When they reach the moon, they are initially placed in jeopardy as their craft lands in a crater and they fall into a subterranean sea inhabited by monsters, the Green Meanies