Synonyms for Cavernous (same or very similar meaning)
WordNet sense 1 (not solid; having a space or gap or cavity):
hollowWordNet sense 2 (filled with vascular sinuses and capable of becoming distended and rigid as the result of being filled with blood):
erectileWordNet sense 3 (able or tending to expand or characterized by expansion):
expansiveFrom the ODE community, based on WordNetadd/edit