Below the mountains that overlook Hong Kong’s financial district, drainage engineer Alex Lau is ankle-deep in water flowing into a giant tunnel almost twice the height of a double-decker bus.

Part of a ground-breaking, $3.8 billion (30 billion Hong Kong dollar) drainage network, this tunnel runs nearly the length of Hong Kong Island and has saved the city from floods that decades ago routinely cost lives and caused widespread destruction.

“This tunnel intercepts about one third of the rainfall for the northern Hong Kong area,” Lau says, as he looks west to the point where the tunnel fades to black. “We have about 34 intakes and all the intercepted water will be diverted into the tunnel and carried all the way to the sea.