Would have thought it would be easier to dig a hole and lay a floor. The interesting thing is that this is smooth concrete like found on the floors inside new homes so it would seem to indicate it is a floor. Also I didn't see any reinforcement rods being laid before the concrete was poured. Might have missed it but I don't think so. Also there is what looks like a small metal cover which can be seen in the photo and which looks like wet concrete. Wonder what that is for?
Also thinking we need input from Constructor.