  A versatile and portable magnetically shielded room with a field of (700 \pm
200) pT within a central volume of 1m x 1m x 1m and a field gradient less than
300 pT/m is described. This performance represents more than a hundred-fold
improvement of the state of the art for a two-layer magnetic shield and
provides an environment suitable for a next generation of precision experiments
in fundamental physics at low energies; in particular, searches for electric
dipole moments of fundamental systems and tests of Lorentz-invariance based on
spin-precession experiments. Studies of the residual fields and their sources
enable improved design of future ultra-low gradient environments and
experimental apparatus.
