When a squirrel chewed through a cable and knocked him offline, journalist Andrew Blum started wondering what the Internet was really made of. So he set out to go see it -- the underwater cables, secret switches and other physical bits that make up the net.  The revelation was shocking. “The Internet is a transcendent idea,” Blum says. “It was unequivocally not something a squirrel could chew on.”  That chewed cable was an “a-ha” moment for Blum, and kicked off two years of reporting for his book, Tubes: A Journey to the Center of the Internet.