  We describe a rigidly-mounted optical cavity which is insensitive to inertial
forces acting in any direction and to the compressive force used to constrain
it. The design is based on a cubic geometry with four supports placed
symmetrically about the optical axis in a tetrahedral configuration. To measure
the inertial force sensitivity, a laser is locked to the cavity while it is
inverted about three orthogonal axes. The maximum acceleration sensitivity is
2.5\times10^-11/g (where g=9.81 ms^-2), the lowest passive sensitivity to be
reported for an optical cavity.
