Cave dwelling, or troglophilic, animals spend their whole lives in cave systems. Living in perpetual darkness, many cave species have lost the sight that their evolutionary ancestors had and become blind. Vestigial eyes can often be seen. They're also usually pale in colour as they don't need to produce skin pigment as camouflage or protection from the sun.
Curious life-forms thrive in poisonous underwater caves.
The hazardous conditions here posed a problem for the equipment as well as the crew. Sulphuric acid eats through plastic so the high-definition camera had to be protected in an underwater housing. The odds seem heavily stacked against there being any wildlife to film here. Yet there are perfectly adapted fish and extremophiles that actually depend on their toxic surroundings.
Fewer than 100 of these cave dwellers remain in the world.
The crew set up specialist, cool lights that operate at low temperature to prevent the water temperature rising suddenly and to avoid any resulting change in the salamander's hunting behaviour. Fortunately, the shoot was successful - the same cave was off limits the very next day following rock movement overnight.
One of the most highly adapted creatures on Earth, Thailand's cave angel fish.
Huge fluctuations in oxygen and CO2 levels 1,000m into the cave had similar effects on the crew to working at altitude. Protective masks and oxygen monitors were vital. A high-definition camera, fitted with a periscope lens waterproof to a depth of about 2m, was used for the real close-ups. Strong water current made it hard for both crew and tripods to stay upright.
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