Think it was an early British Sci Fi serial (or perhaps a one off) from the late 1960's (or more probably) early-mid 1960's, may have been in black and white.
What I remember most is at the end where a group of scientists (at least one man and one woman) trying to leave and block up a cave that seemed to be either a door way to the past (or to a hidden world).
The reason for this was a prehistoric monster (something along the lines of a Tyrannosaurus Rex?) was trying to escape.
Because it was live action, the monster's roar was worse than its bite.
I've checked both "Out of This World" and "Out of the Unknown" but it doesn't appear to be from either of these series. May have been an "Armchair Theatre" production?