A British version of Godzilla.
Treasure hunters in a ship off Ireland are nearly sunk by an underwater volcano eruption.
After going to a nearby island for repairs, they encounter and capture a 65 foot tall monster called Gorgo (who strangely resembles Godzilla). They take it to a circus in London.
We discover that Gorgo has a 200 foot tall mother who proceeds to trash anything between her and Gorgo.
After destroying Navy ships, Tower Bridge and Big Ben, Gorgo and his mother are reunited.
They go back to the sea in what is viewed as a pro environment ending.
Thanks, Don P.!