Just an hour away from Ljubljana, lies one of the most spectacular natural sights you will ever lay eyes upon. The huge crowds at Postojna Cave do little to take away from the jaw-dropping wonder of this vast, two million year old karst cave system. There’s even a concert hall located deep underground that can hold 10,000 people. Dress warmly and prepare to be awed. Also if you’ve got a way to get to nearby Predjama Castle, built partially inside the Postojna complex, it is not to be missed.