Geograph - photograph every grid square

Yes provided they are accurately located and meet the usual criteria.

If the underground space is artificial such as a mine or railway tunnel, they can only be classed as "accepted", i.e. tagged as "inside" or "close look", rather than qualifying as a "Geograph".

If the underground space is natural such as a sea-cave, limestone cave etc., then if there is a large enough space in view a Geograph status is possible, but in a small space it would still be accepted as a "close look".

With the exception of straight tunnels, getting an accurate position underground in the absence of a GPS signal and without a detailed map of a cave system is tricky. In many cases it may be best just to give 4-figure grid reference (i.e. not try to locate the photo within the square).