I think there are two islands. I seem to recall that one of Danielle's maps depicted two islands, one smaller than the other.
If that is the case, then I'd like to know what things are on this second island.

As for the idea of some kind of tunnel network, I wouldn't be surprised if that was correct. The Blast Door map seems to depict tunnels that all point in the direction of The Pearl, but each one on the map has a cross at the end of the "tunnels", perhaps showing that the tunnels had become blocked up somehow.