I'll Be a Mummy's Uncle - S3.E23

With plans to build an indestructible empire, King Tut goes in search of Nilanium, the world's strongest metal. But with the only known deposit lying directly beneath Wayne Manor, Tut's tunneling soon leads him to the discovery of a lifetime - the Batcave itself. With their secret identities in jeopardy, Batman and Robin race to stop him before his find is leaked to the world.