Why dont we say "Il y a deS belles montagnes dans le Jura." ?

Yes, that's what you'd expect, normally. But when there is an adjective between des and the noun, turns into de. Gotta love the French and their language full of exceptions! :)

Here is the corresponding lesson: "Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles)

Aha, that link is for a B1 lesson! No wonder I thought it was an error too. :) Thank you Chris.