Nope, we’re not talking about the Aberdeen Tunnel or the Cross-Harbour tunnel. From Lamma Island to Aberdeen to the New Territories — these little holes in the hills are everywhere.

Some of the tunnels were dug by the British leading up to World War I, like the Gin Drinkers Line in Kowloon, built to defend Hong Kong’s main source of water, the Shing Mun reservoir. This subterranean matrix was doubly confusing, as the narrow alleyways were modeled off London streets to make it easier for the British to navigate, and harder for enemies.

The intricate underground network expanded while the Japanese occupied Hong Kong for nearly four years in the 1940s. The Japanese dug out a variety of trenches in order to avoid, confuse, hide… and store explosives. The war, luckily, never called for the guerrilla tactics despite a bloody period of fighting and shelling when the Japanese first arrived.

During this time, hundreds of people took shelter in air-raid tunnels that had been built by the British. There are various networks of these shelters all over Hong Kong — even downtown on Queen’s Road Central across from HSBC, you’ll find a filled-in portal that was used during WWII.

By now, most of the tunnels have been filled in or just kind of left to collect spiderwebs and lots of bats. Hiking Wilson Trail and up to Jardine’s Lookout, you’ll pass by several — most of which are boarded up with threatening-looking wooden planks that seem to get the job done.

If you’re keen to hunt down the other hundreds of tunnels, check out Gwulo.com‘s extensive map.