aka Jules Verne's The Fabulous Journey to the Center of the Earth
How bad is it? It's hokey and campy and overly-familiar.
Should you see it? It's not quite good enough or bad enough to recommend.
This is a cheap version of The Land That Time Forgot, of a generation earlier, with an added love interest and, for some reason, a time-traveling scientist thrown in half-way through. The discovery of a manuscript causes people to try to recreate the voyage to the center of the earth described in the manuscript. The dialogue is unintentionally campy, but gets annoying. There are sea monsters that are just hand puppets. There are back-projected rubber dinosaurs, giant turtles and a big ape. It's a bit sluggish and episodic and the introduction of Olsen, the time-traveling scientist they meet, is confusing (though it does help to neatly wrap up the ending).