  A new axiom is proposed, the Ground Axiom, asserting that the universe is not
a nontrivial set-forcing extension of any inner model. The Ground Axiom is
first-order expressible, and any model of ZFC has a class-forcing extension
which satisfies it. The Ground Axiom is independent of many well-known
set-theoretic assertions including the Generalized Continuum Hypothesis, the
assertion V=HOD that every set is ordinal definable, and the existence of
measurable and supercompact cardinals. The related Bedrock Axiom, asserting
that the universe is a set-forcing extension of a model satisfying the Ground
Axiom, is also first-order expressible, and its negation is consistent. As many
of these results rely on forcing with proper classes, an appendix is provided
giving an exposition of the underlying theory of proper class forcing.
