Its a cave floor. This surface may be [[prospecting|prospected]] and mined for [[stone shards]]. Repeated [[mining]] will gradually lower the height of your nearest corner by 1 dirt/slope per action. You can raise corners of cave floors by using [[concrete]], or having a [[mag]] priest cast Strongwall and then re-mine the created rock tile.
The following types of [[Resource nodes]] can spawn on Cave floors: [[Mineral debris]], [[Rubble]], [[Lichen]] and [[Fungus]].
= Cave ceiling =