How to simulate caves?

[carp]

Does anyone have any good tips for simulating cave and tunnel entrances? I'm trying to make some Okinawa/Iwo/Saipan maps. In the past I've used a lower elevation with a fortification in it, like maybe having an elv=3 and using the edge with an lv=2 for the fortifiction This protects the lower elevation from less than direct hits by arty but greatly restricts the ability for weapons use within and by the fortification. I'm not asking for a new icon, just a work-around. Cave and tunnel entrances should be more resistant.

Thanks