Circumpolar! is a novel by Richard A. Lupoff published in 1984.
Circumpolar! is a novel in which an alternative history is depicted with vastly different geography: A highly flattened disklike "donut" topology. It describes a Great Air Race to circumnavigate the Earth by traveling through the "polar hole" (hence the title), around the "underside" of the Earth to its "rim", and back to its starting-point. The race features the von Richthofens competing against Charles Lindbergh and Howard Hughes.
Dave Langford reviewed Circumpolar! for White Dwarf #70, and stated that "It goes on a bit long and it grossly libels the Red Baron, but it's amusing."
In his ', Brian M. Stableford cited the book as one of the relatively rare examples of science fiction dealing with the Flat Earth concept.