One thing about LBT is that they are really not thrifty with the word mysterious even in places where there doesn't seem to be much of a mystery.
The mysterious beyond is where many of the Great Valley's inhabitants used to live before they came to the Great Valley. What makes it so mysterious? And would migrating dinosaurs find anything mysterious about it?
Not much of a mystery in case of the mysterious island either as far as I can see.
The German title of "The Secret of Saurus Rock" is "Der geheimnissvolle Berg der Saurier" which translates "The mysterious Mountain of the dinosaurs". In a sense there is some mystery about that place. Are those teeth around the rock's neck real teeth? How much substance is there to the legend of the lone dinosaur? I have not watched the TV series episode which apparently disenchants the mystery of the lone dinosaur (identifying Doc as the one without any doubt), but there is not really too much of a mystery about the lone dinosaur anyway. A lone dinosaur who does what other LBT dinosaurs had done before...
Come to think of it this last statement is not quite true. In the original movie Littlefoot's mother payed with her life for defeating the sharpooth in a one to one fight. In LBT 2 Littlefoot's grandpa was defeated and only saved by the intervention of a flyer (presumably Petrie's mum) when he took on a sharptooth all by himself. Chomper's parents were defeated only when they were outnumbered. In LBT 3 it is again a large group of leafeaters against a smaller group of sharpteeth rather than a one to one fight. So LBT 6 in fact was the first time a single sharptooth was defeated by a single leafeater (sharpteeth began to degenerate from there on).
Anyway, I'm straying from the topic.
It seems that the German translations make more frequent use of mystery and secrets than the English original versions. LBT 11 is "Das Geheimnis der kleinen Saurier" ("The secret of the little saurians"). I don't see a big secret in the story except perhaps that they are hiding from the larger dinosaurs for reasons that were never explained. There was not exactly an invasion in that movie either though
