The Cave of Swallows in Mexico is a 400 metre vertical shaft. Its deep enough to engulf the Empire State Building yet few people even know its existence.
Lechuguilla cave, in the USA, is 193 kilometres long and 500 metres deep. It contains whole chambers filled with the most astonishing crystals including some a staggering six metres long. There's nothing like it anywhere on earth yet the name Lechuguilla is virtually unheard of.
Caves are Planet Earth's final frontier and this programme will go where few have been before.
Caves are one of the only habitats not directly driven by sunlight but this doesn't mean there's no wildlife.
Cave angel fish are perhaps the most adapted creatures on earth since they live only on cave waterfalls - hanging on with microscopic hooks on their flattened fins. Until now they have never been filmed before.
Deer Cave in Borneo is a daytime retreat for five million bats and their droppings support an entire community of creatures. Shine a light on one massive pile of droppings and the whole place shimmers with millions of dung-eating insects.
Planet Earth descends into an undiscovered world to introduce some of the most remarkable and bizarre animals on earth.
From cave swiftlets who navigate through pitch black caverns using echo locations and build nests out of saliva, to the troglodytes - weird creatures that never see daylight or ever set foot on the surface.
Specialists like the Texas cave salamander and Thailand's cave angel fish have neither eyes nor pigment. The entire populations of both are found in just a couple of caves.
Gaining unique access to a hidden world including poisonous caves and flooded caves, full of stalactites, stalagmites, snotites, and troglodytes, few natural history programmes could boast such a wealth of surprises.
Producer - Huw Cordey
A Beautiful Journey Underground - Lechuguilla Caves, USA
Could there be a greater contrast? From the vivid blue sky above the desert we were walking over to the subterranean world beneath our feet that we were about to enter?
Clinking across the desert in our tight fitting caving harnesses it was amazing to think that below us was one of the longest, deepest, and most beautiful caves in the world. A place so extraordinary that, since its discovery in 1986, it has become the Holy Grail for all serious cavers. Its name - Lechuguilla.
Unfortunately for the world's cavers, Lechuguilla's fragile beauty has made it the most restricted cave on earth - open only to specialist scientists and those mapping the ever increasing passages.
We'd spent two years negotiating for permission to film in Lechuguilla, but after finally winning them over they'd made it clear that this was likely to be the last crew to be allowed to film in the hallowed caverns. The pressure was on.
To capture Lechuguilla's unique beauty, the crew were going to have to spend ten days underground - the maximum time we thought we could work without sunlight while still retaining our sanity.
Return trips to the surface were just not possible since getting to our underground campsite involved a gruelling eight hour journey, which passed through such narrow passages we were worried about getting ourselves through them - let alone the small jib arm vital to our filming.
The first half of the journey was unremarkable and certainly gave no indication of the beauty that lay ahead, but once we'd descended Boulder Falls, an adrenaline fired abseil of 60 metres in utter blackness, we began to get a glimpse of why we were here.
Glacier Bay Cavern was well named. The floor looked as if it was formed from huge chunks of carved ice. It was quickly followed by other chambers whose names neatly sum up the feelings of the first explorers - places like Snow White's Passage, Tinseltown, Land of Awes and Prickly Ice Cube room.
While all the decorated caverns we'd seen were undoubtedly spectacular nothing could quite prepare us for the Chandelier Ballroom, where six metre long collections of hand sized crystals hung from the ceiling.
The sight was utterly other-worldly. I felt as if I'd been miniaturised and stuck in a large empty freezer compartment.