Mysterious and often deadly, sinkholes are typically formed when the land above the surface collapses or sinks due to water erosion which causes the ground to crack and fall away. Global warming and volcanic activity can also cause them to emerge. From mystical blue holes and shimmering swimming spots to sprawling underground caverns and plunging pits, sinkholes can be as beautiful as they are terrifying. Here we take a look at some of the most incredible sinkholes around the world, in ascending order of depth – where measurements are known.