The tailrace tunnel of Niagara Falls

Holy crap! "Behind the raging horseshoe falls of Niagara there lurks a dormant monster, a century old redbrick tunnel painstakingly laid. There is no recorded tally of its human cost but in 1906 it would be the biggest tunnel of its type in the world. Like the secret hideout of a supervillain it defies belief and comprehension, a stronghold behind the crashing waterfall."