Do ruins always have a basement?

[MagpieOAO]

I've only just discovered this and have been pleasantly surprised to find a few, but some especially partially submerged ones, I'm not sure.

 

Also is there any point to digging around altars? The big 6x6 rock tables with arches around them. I tried but didn't find anything =/

[Thorfinn]

I have an almost perfect record. If I dig out a ruins, it has no cellar.

[Aksha]

No, not all ruins have basements. And basements vary in the number of rooms and things. Most often, the walls of underground rooms are made of dry masonry, so if you find dry masonry blocks while digging in the ruins, you can consider that you have found a basement.