A prehistoric hidden cave and hippo bones found beneath a  Welsh castle could transform understanding of ancient life in Britain, researchers said Thursday, calling the site a "once-in-a-lifetime discovery." Previous small-scale excavations at Pembroke Castle – the birthplace of Henry Tudor and a popular tourist attraction – have already uncovered evidence of early humans and animals, including a now extinct hippopotamus that roamed Wales 120,000 years ago. A major five-year archaeological