THE EERIE ICELANDIC LAVA TUNNEL
Not far from Reykjavik lies one of Iceland's buried treasures - the Raufarhólshellir lava tunnel, which is well over 5,600 years old and stretches for more than a kilometre. Formed in the aftermath of vast and hellish eruptions, which saw a flow of ancient lava slowly eat its way through the rock while leaving a hardened crust around it, the tunnel is a magical, other-worldly realm. Natural skylights, created when parts of the tunnel caved in, let in shafts of light as well as snow, which accumulates and can stay heaped up even in the summer.
Intrepid explorers who enter the tunnel will find themselves literally walking in a path carved out by the molten, fiery innards of the Earth. The exposed minerals in the rocks give the tunnel an almost psychedelic hue, with reds and yellows blending into each other all around, while encrusted ice crystals glitter like jewels. It's no wonder the tunnel has become an almost mythic point of pride for Iceland, drawing in those eager to come face to face with its hidden splendour.
LONDON'S HIDDEN SHELTERS
Dotted around London are unusual, circular buildings which barely register on the radar of most passers-by. There's a pair of these peculiar pill-box structures amid the bohemian hubbub of Camden Town; another pair can be found by the green sprawl of Clapham Common. Even those who clock the buildings will have no idea they're actually entrances to deep tunnels, each with enough space to hold 8,000 people, along with bunks, medical facilities and kitchen areas.
These largely-forgotten shelters were built in the early stages of World War Two, to protect Londoners from the inferno of the Blitz. Eight such shelters were constructed, with each one comprising two large tunnels divided into a pair of decks. The Blitz was actually over by the time the shelters were completed, though they did come into use when the V-1 flying bombs and V-2 rockets began raining down on the city later in the war. In the run-up to D-Day, the shelters at Goodge Street in central London were even used as the headquarters of General Dwight D. Eisenhower (who would later become US president).
In 1948, the shelters under Clapham South temporarily housed some of the first immigrants to arrive in the UK from the Empire Windrush. Since then, some of the shelters have been requisitioned for storage purposes, while in Clapham one of the shelters has been turned into an underground farm growing micro herbs and salad greens, and providing tours for curious foodies.
THE DEADLY MAZE OF ODESSA
Perched on the shoes of the Black Sea, the city of Odessa is one of Ukraine's holiday hotspots, famed for its ornate Mediterranean-style architecture. Yet one of its most striking attributes is hidden from view: the Odessa Catacombs, a sprawling subterranean labyrinth, with tunnels that are estimated to be 1,600 miles long.
Hundreds of years old (its exact time of origin is hard to pinpoint for certain), the maze is so vast that it's not yet been fully mapped. It was originally carved out to extract the raw materials to build the city itself. There are said to be around 1,000 entrances to the Catacombs, which were used as a hiding place for resistance fighters when Odessa was occupied by the Axis powers.
Since then, the Catacombs have attracted daredevil explorers and inspired eerie stories - most notoriously, the case of a girl named Masha who allegedly got lost there after a night out in 2005, dying from cold or dehydration in the maddening maze. The story has since been dismissed as an urban myth, though speculation still abounds about people who may have died under Odessa.
THE GORGEOUS CISTERN OF ISTANBUL
Straddling the divide between Asia and Europe, Istanbul is adorned with marvels like the Hagia Sophia and the Blue Mosque. Equally sumptuous is an underground realm which beguiles anyone who enters. This is the Basilica Cistern, a Byzantine wonder which lies only a short stroll from the Hagia Sophia.
Built in the 6th Century as a reservoir for the city above, this vast cistern - which was constructed by an army of slaves - is supported by hundreds of marble columns, some of which feature carvings of the head of snake-headed gorgon, Medusa. Ome of the columns is adorned with tear-shaped patterns, said to represent the weeping of the anguished workforce. All of this gives it the solemn atmosphere of a temple, with amber lights casting a soft glow over the intricate columns and placid waters, through which small fish swim.
It's no wonder it's been used as a filming location - most notably in the classic Bond film From Russia with Love, as well as the adaptation of Dan Brown's bestseller, Inferno.