The entrance of the old abandoned mine is a 45-minute hike from the Tanygrisiau base near Blaenau Ffestiniog. That's before you can enter the mystical caves.
After putting on the safety helmet, miner's lamp, harness, and Wellington boots (classic style waterproof boots made of rubber), you are ready to enter the underground world.
Although the modified route is much safer than before, it still takes an hour to trek to reach the underground hotel.
Along the way, you can see the various relics of this 19th-century mine, old mine cars, decaying tracks, and bridges.
Go Belo's five, six, and seven-hour-long adventures vary in difficulty and depend on how much adrenaline you want to run through your veins.
While the starter tour with a minimum age requirement of 10 has relaxing boat trips on deep blue lakes, the most extreme offer is only for adults and includes (secured) free falls and a run on the Goliath zip line.
The course is run by professional and friendly hosts, with safety being their number one priority. The guides' "adventure starts where daylight ends" t-shirts forecast an experience you will not likely forget.
Some sections also require rock climbing skills.
Across the mine, suspended cables connect parts to get you safely (and really fast) from one side to the other. The nail-bitingly exciting zip lines glide over lakes and black abyss so deep you can barely see the bottom.
The majestic subterranean world has caves as large as in Europe's most famous salt mine in Wieliczka, but without the decorations.
During your journey to the underground, you will see rusty mining equipment and even water (or spirit?) bottles of Victorian time workers.
The world's longest underground zipline
The aptly named Goliath is the longest zip wire that runs in a cave spanning 420 feet (130 meters).
If you want to hear the echo of your screams and shouts, you should opt for the Ultimate Xtreme adventure - the only one to offer a ride on the Goliath zipline.
The world's deepest hotel
The company's founder, Miles Moulding, came up with the brilliant idea of creating the world's deepest hotel in the Cwmorthin quarry. Miles is an outdoor adventure enthusiast who thought setting up a cave hotel deep underground would make perfect sense for two reasons.
First, after a long day exploring the caverns, you wouldn't need to return home but have a room for the night on the spot.
And second, the cave's depth meant it would break a record and become the world's deepest hotel. Now, that's a title you can brag about!
So, after careful planning and setting up new and more accessible routes, he and his team built a log cabin hotel for ten people at the bottom of the mine.
Unlike in other cave hotels, there is no elevator that will take you down. Part of the experience is the hour-long journey through tunnels and shafts carved by miners to reach the site of Deep Sleep. On the only way to the hotel, you must slowly descend the newly rebuilt rope ladder.
Once you arrive at the camping base, you are welcomed by a hot beverage, and the team provides you with an expedition-style meal (which can be vegan or vegetarian upon request).
When you are finished with the food and chatting at the picnic table, you can retire to your comfy room and enjoy the deepest sleep you have ever experienced.
The beds have thick blankets to keep you warm, as the temperature at this depth is only 10° Celsius (50° Fahrenheit).
The underground hotel is open only once per week on Saturday nights.
Depending on your budget and taste, you can choose to sleep in a lodge-style cabin or a grotto with raw rocks surrounding you and the soft light of the candles.
The morning starts with a wake-up call at 8 am, and after a quick breakfast, the staff will guide you back to the surface, so you can go and tell your friends and family that you slept in the world's deepest hotel!
The deepest cave hotels
So, how does Go Below's cave hotel compare to the others?
The Grand Canyon Caverns Suite in Peach Springs, USA, has a room 220 feet (67 meters) underground.
Sweden's Sala Silvermine is home to a bedroom carved into solid rock 509 feet (155 meters) below the surface.
That adds up to 866 feet (264 meters) difference between the world's first and second deepest hotel.
To put it into context, if the 1,375 feet (419 meters) Deep Sleep was not underground but in the sky, it would be as tall as Jin Mao Tower - the 35th tallest building in the world. In British, that's 3.5 Big Bens stacked on each other.