The Rat Hole is sort of an igloo-shaped, clandestine building mainly constructed in pure metal. It lies somewhere inside a junkyard presumably located at the outskirts of the city, where its owner, Techrat, can get away from the rest of the world and find the solitude he so eagerly craves for in order to work on his technological projects without any interruptions.
The junkyard the Rat Hole is located at is ideal for Techrat's schemes, as it provides him with all sorts of materials and replacement parts for his electronic gadgets in an almost unlimited supply, and it also provides enough room to make all sorts of tests, which more often than not happen to be quite dangerous.
The place has been seen going through drastic changes as the series progressed, being at the beginning just a common warehouse in a vacant lot, to the metal igloo mentioned above. Whether this place actually evolved or if it was only the designer in turn own interpretation is not confirmed.