I've started thinking in shades of grey.

You don't have to go straight from the 711-concrete-grid to some yuge underground paradise.

There must be "intermediate towns and islands"... bridge areas.

Places that are pretty much off-grid, but still on the surface of the EArth. So they retain strong links to Lemuria, but are still technically on the surface.

There must be tons of cross-civilization interaction on these islands.