OGC standards for 3d geometries?

Dear spatial data friends

Just thinking about some spatial data in a city context and looking for
pointers to the recommended approach or state of the art for representing
3D geometries in the spectrum of OGC standards?  Presumably you can define
a surface representing the 'top' of buildings, and another surface defining
the 'ground'. What do you do if you want something more complex? Perhaps
floors within a building, or surface, tunnels, flyovers etc?

Thanks in advance for any good links or advice

Best regards

Bill