Mud tank

A mud tank is an open-top container, typically made of steel, used as a reserve store for the active circulation of the drilling fluid on a drilling rig. They are often called mud pits, which comes from the fact that they used to be nothing more than pits dug out of the earth.
The tanks are commonly open-top and will have walkways on top of them to allow traversing and visual observation of the drilling fluid and to monitor the level of fluid in the tanks. The walkways also allow access to other equipment mounted on top of the mud tanks. More recent offshore drilling rigs will have closed in tanks for safety.
For a typical petroleum drilling rig there are normally 2 tanks. Each tank is sectioned off into smaller separate compartments designed for more specific purposes, such as a settling tank (sometimes called a sand trap), used to allow solids such as sand to settle out of the drilling fluid before it flows into the next compartment. Other compartments will have agitators (which are large fan blades) that stir the fluid to prevent the chemical constituents of the drilling fluid from settling out.