ColorChallenge / Petrovaradin Undergrownd / Tuesday Orange
Did you know that under Novi Sad fortress you can find almost 20 km long underground tunnels. It's four level tunnel structure made in 18th century. Now days you can visit with guide part of it, and it can be very interesting and unusual tour to take.
And as it looks here, very mysterious one..
Here is my submission for Tuesday Orange, contest initiated by @kalemandra.