A type of tank engine in which the water tank is situated between the locomotive frames; thus, conceptually, in a 'well' between them, below the crew.
The good thing about this design is that the center of gravity is kept low, avoiding one of the biggest problems with the tank engine. However, there's only so much space there, so only small locomotives have had this design. Apart from very old locomotives, only small narrow gauge steam locomotives tended to use this design.