Sometimes you have to go underground to see some of the coolest sights in the world.
Humans have always astounded themselves on their feats of genius engineering, and the ability to bore tunnels into massive mountains or underneath sprawling cities is high up on the list.
WATCH: Get your adrenaline fix over at the world's 10 wildest amusement park rides
Tunnels have multiple uses - for transport, protection, homes and even as burial sites - and the ones of old create unique urban exploration expeditions for the travellers of today.
South Africa itself holds many mysteries and secrets in its hidden tunnels - especially in Cape Town - and around the world there are hundreds of tunnel networks ready to be explored by those who won't miss the sun and fresh air too much...
Here are some of the top tunnels to check out in SA and the world.
Cape Town's Tunnels, South Africa
Cape Town has a crazy underworld that few people know about. There are tunnels that run under the city where water from Table Mountain run out to sea, there an abandoned tin mine with tunnels that bore into the mountain itself and old water tunnels that run all the way through the mountain after doing a hectic hike through Disa Gorge.
If you want to check any of these out, you'd have to take a guide with you as it could get pretty gnarly, you'll need permits for some of them and you could easily get lost. Good Hope Adventures is one of the top Cape Town tunnel experts, so check out their website to organise a tour.
WATCH: Teaching 100 kids the value of history in the spirit of Mandela
Cullinan Diamond Mine, South Africa
Did you know that you can explore one of the most famous diamond mines in the world, right here in South Africa? This mine in Gauteng produced the largest diamond in the world and is currently featured in Britain's Crown Jewels.
You can take a 4-hour underground tour through its tunnels, but remember that it's still a working mine and no cell phones or children under 16 are allowed on the underground tour.
Kimberley's Underground World, South Africa
You can also check out South Africa's other famous mine - Kimberley's Big Hole - and take a detour underground. You'll get dressed up in typical miner gear before you plunge 840 metres into Mother Earth.
You'll also get to chat with real miners about their lives and the history and present of the diamond industry.
READ: What to do this summer in the 9 off-the-grid cities of SA
Gotthard Base Tunnel, Switzerland
This railway tunnel in the Alps is the longest tunnel in the world - it's about 57km long. It forms part of a larger project to create more direct routes through the Alps and this section took 17 years to build before opening in 2016.
You can go through the tunnel by taking the EC11 express train from Zurich to Lugano.
PICS: After 17 years, the world's longest rail tunnel opens
Catacombs of Paris, France
This is probably one of the most famous tunnel networks in the world - and the spookiest. Millions of people's remains are housed in tombs right under the city of Paris and the tunnels were made by quarry miners before they started using it as a graveyard.
There are many tours offered through these creepy hallways, and if you know the right people you could even take a dip in one of the unmapped pools.
Siloam Tunnel, Jerusalem
Also known as Hezekiah's Tunnel, this is an ancient water channel that runs between Gihon Spring to the Pool of Siloam underneath the City of David in Jerusalem.
The tunnel is quite claustrophobic, and you should be prepared to get wet and bring flashlights as there are no lights inside the tunnel.
READ: Holy Travel: Franciscan museum in Jerusalem shows life in Jesus' time
Chu Chi Tunnels, Vietnam
This intricate network of tunnels was built and used by local soldiers during the Vietnam War. It protected them from bombs and included underground villages, meeting rooms and hospitals where the soldiers stayed during the day while conducting military operations at night.
It's situated in Ho Chi Minh city and forms part of a war museum. The spaces are quite small for foreigners and it involves crawling around a lot.
Cappadocia is full of underground cities, and the deepest of them all is Derinkuyu where about 2 000 people lived. One of the unique sights to see is the underground church.
Only about 10% of the city is open to the public, and if you want to know more about its history it's advised to go with a guide.
QUICK GUIDE TO TURKEY: Easy eVisa travel for South Africans
Pilsen Historical Underground, Czechia
The tunnels under Pilsen were built in the 14th century, once serving as storage space for food and beer - and even rumoured to have hidden treasure!
A guided tour will take you past the ice cellar, a water tower and even a medieval bookbinding exhibit.
Find Your Escape by searching and comparing flight prices here. You can also sign-up for the Traveller24 Weekly Newsletter – Subscribe here. Or download the News24 App here, to receive expertly curated travel ideas and deals directly to your mobile.