The artist kind of failed in this section, but it's not really a piece of land. At the last panel Souma calls it "Aircraft Carrier". If I recall the source material correctly, what they did was build an actual "flying aircraft carrier" warship using the giant Susumu-kun Mark V which was also shown in the anime. And then they solved the issue with wyverns not wanting to roost in the sky or at sea (basically ruining the point of a flying warship) by making a man-made cave external to the "hangar". They kinda took the Ghibli-esque approach here.