In Borderlands 2 Safe and Sound, Marcus tells us he lost his safe when Sanctuary took off the ground, and he wants us to go back to Sanctuary Hole and retrieve it.
During the assault on Sanctuary, Marcuss personal safe was blown of his store, landing somewhere outside the bounds of the city. He has asked you to go to Sanctuary Hole so you may return the contents of his safe to him.
Item: Eridian Artifact / Shotgun
Experience: 3859 XP
In order to reach Sanctuary Hole, we need to fast travel to Three Horns Divide first. From here we can take the exit point to Sanctuary Hole (the area where the town of Sanctuary was). Then, we have to go to the waypoint to search for Marcuss personal safe.
Our secondary objective is to kill 25 Sanctuary squatters (these are actually all the enemies found in this region).
When we are close to the safes location, the bandits will drop it into the giant hole that leads to Caustic Caverns. To reach Caustic Caverns we have to drop down to the lower levels, paying attention where we land (at the end of the map there is an elevator, but if this is our first visit here, the chances are it may be lowered already).
After that, we make our way through the small enemy installation, and at the other side we use the elevator to arrive at the exit point to Caustic Caverns. Once inside, we open the large blast doors using the rotating wheel.
This place is filled with varkids, but we can also find volatile crystalisks. Their only weak points are the crystals on their legs (unless we use an AoE weapon, like a rocket launcher).
But the most important thing here is to avoid the acid ponds on our way to the eastern side of the area, where the waypoint is. Once we get to the safe, we can see that it is actually on top of a huge crystalisk named Blue.
The same strategy we used on other volatile crystalisks works with him too; we just shoot the blue crystals on his legs using our most powerful weapons (note that some explosive crystalisks may spawn).
When Blue is down, we open the chest. Instead of money, inside we find some pictures with Moxxi, so we are presented with a choice: we either return the pictures to Marcus for a shield, or we give them to Moxxi for a shotgun.
Regardless, picking one of them will complete the Borderlands 2 Safe and Sound mission.