One of special-effects wizard Ray Harryhausen's pre-Seventh Voyage of Sinbad efforts, 20 Million Miles to Earth borrows a few pages from King Kong. An American spaceship crashlands off the coast of Sicily. The rescue party discovers that the astronauts have inadvertently brought back a curious gelatinous mass from the planet Venus.
This lump of goo rapidly evolves into be a living reptilian creature, which scientists label an Ymir. While being subjected to laboratory experimentation, the Ymir begins growing by leaps and bounds, and before long the gigantic monstrosity has escaped and is wreaking havoc in Rome.
After battling a zoo elephant and taking a swim in the Tiber, the gargantuan creature holes up in the Colosseum, where the film's pyrotechnic finale occurs.