flat as a pancake

Extremely level, especially too much so. For example, There are no hills; this terrain is flat as a pancake. This simile dates from the 1500s and has survived its contemporary, flat as a flounder. It is sometimes used, either disparagingly or ruefully, to describe a small-breasted woman.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.