The Doctor laughed with unhidden amusement. "Oh, now, that's not really so odd, is it? Humans lived in caves long before they learned how to build houses." He failed to notice, let alone correct, his pronoun use. "It's either a cave or some underground complex of metal walls and small, sterile rooms. I rather prefer the cave. Far less confining." And less like a lab, where they'd be the test subjects.