The Collapsed Observatory is a GW2 jumping puzzle located in Kessex Hills. It’s fast and easy, though you will need to fight a Veteran Troll at the end to claim your reward.
Start by going to the Cereboth waypoint toward the southeast of Kessex Hills. You’ll be running south into a cave to start the jumping puzzle.
The jumps in this puzzle are easy. It’s mostly about finding where you’re supposed to be jumping. Follow the images below until you reach the veteran troll which is located in a short drop near the chest.