  We describe a method to create and store scalable and long-lived entangled
spin-squeezed states within a manifold of many-body cavity dark states using
collective emission of light from multilevel atoms inside an optical cavity. We
show that the system can be tuned to generate squeezing in a dark state where
it will be immune to superradiance. We also show more generically that
squeezing can be generated using a combination of superradiance and coherent
driving in a bright state, and subsequently be transferred via single-particle
rotations to a dark state where squeezing can be stored. Our findings, readily
testable in current optical cavity experiments with alkaline-earth-like atoms,
can open a path for dissipative generation and storage of metrologically useful
states in optical transitions.
