HB: Similar to soakhole_pnt with no water going into it, or just generally a karstic area? No attributes to worry about, so just natural=sinkhole?

see:
http://wiki.openstreetmap.org/wiki/Tag:natural%3Dsinkhole
http://wiki.openstreetmap.org/wiki/Proposed_features/sinkhole

HB: approved for Antipodes and Antarctica 21 Aug 2012. Seems to be ready for the mainland.