Mouse and Rat Hole
The difference between the mouse hole and rat hole is as follows:
The mouse hole is an opening in the rig floor that allows for the fast connection of a single joint of drill pipe to the drill string during drilling. A new joint of drill pipe is lowered into the mouse hole where it waits for the next connection. Each time the drill bit has deepened the hole (the length of a joint of drill pipe), an additional joint of drill pipe will be added to the drill string. The kelly will be unscrewed from the drill string and screwed into the new single joint of drill pipe. The kelly and drill pipe joint will be raised from the mouse hole and screwed into the top of downhole drill string so that the rig can drill deeper. The rat hole is also the extra hole drilled after total depth has been reached. It allows for more working room for long or multiple tools downhole. It also is used to give additional room for junk and sand to fall.
The rat hole is an opening in the rig floor where the kelly can be stored. The kelly fits into the rotary table. The rotary table turns the kelly which is screwed to the top of the drill string.