On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled The Great Escape. You can begin this quest once you gain access to the elevator which leads to the roof of the construction during the main mission named Toil and Trouble.
2) Hyperion Beacon
Ride the elevator up to the roof, where you will find Ulysses . You will have to retrieve a beacon using which Ulysses will be able to leave Pandora. Head back to the lower level and right beside the elevator, on the right, you will see the above building. On its roof there's Frederick the Fish who will grant you additional experience.
Head to the spot marked as  and head down the ladder shown in the screenshot. Below there will be another ladder, use it as well. Pick up the Hyperion Beacon and return to Ulysses . Place the beacon in the designated place beside Ulysses and speak to him to end the mission.