The hollows appear to have formed relatively recently. The next question is, what lies inside? Phys.org writes:
A recent image acquired by NASA’s MESSENGER spacecraft shows the interior of Eminescu, a youngish 80-mile wide crater just north of Mercury’s equator. Eminescu made science headlines last year with MESSENGER’s discovery of curious eroded blotches called “hollows” scattered across its interior and surrounding its central peak, and now it looks like the spacecraft may have spotted some of these strange features in their earliest stages of formation along the inner edge of the crater’s rim.
First announced in September 2011, hollows have now been identified in many areas across Mercury. The lack of craters within hollows indicates that they are relatively young. It was suggested that they may be the result of an ongoing process on Mercury.