What's a sink hole property?

[Marcus Brown]

I'm doing due diligence on a property and on the assessor's property report card, in the notes section it says “Drainage/Sink hole”. It’s rolling terrain but the parcel doesn’t appear to be at the bottom, and there are buildings all around it, with city sewer on site. I emailed the assessor about it and he replied “Field reviewers may have noted reason for conditioning the land value.” Anyone experience anything like this? I have no idea how to proceed and I don't know the area. It's a 2 acre parcel, seller is letting it go for $5500.