On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Hell Hath No Fury. You can start this quest in the Sanctuary location after completing the main mission named The Once and Future Slab.
2) Foreman Jasper/Supply key
Speak with Moxxi  and travel to Opportunity with the Fast Travel. If you haven't activated the device yet, move to The Highlands Hyperion Bridge. Afterwards go to the bridge in the western part of the map, where you will find the passage to Opportunity. There, find and kill Foreman Jasper  and pick up the key which he drops.
Head to the container marked as  and open it. Inside you will find Foreman's Explosives. On the way to the place where you're supposed to plant it, you will get attacked by a Constructor and Robots.
Head below and place the explosive on the dam (you need to jump and quickly press Use). Head up, using the ladder on the right. The charge will soon afterwards explode and water will flood the building site. Return to Sanctuary and speak with Moxxi  to finish the mission.