1) Drilling technology 2) Fishing community in England 3) Home for Speedy Gonzales 4) The burrow of a mouse 1) Opening 
[drilling] The mousehole is the storage area on a drilling rig where the next joint of drilling pipe is held until needed. This hole is in the floor of the rig, bored into the earth for a short ways, and usually lined with a metal casing known as a scabbard. OSHA Definition ...
• (n.) A hole made by a mouse, for passage or abode, as in a wall; hence, a very small hole like that gnawed by a mouse.
Mouse'hole` noun A hole made by a mouse, for passage or abode, as in a wall; hence, a very small hole like that gnawed by a mouse. 
the burrow of a mouse. · the entrance to a mouse's burrow. · a small hole resembling this.