Ljubljana, SloveniaImpressive underground canyon! This world known natural site, on the world heritage list of UNESCO, is one of the most beautiful cave I have ever visited.
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Mammoth Cave in Kentucky is the world's longest known cave system, with more than 400 miles explored, and one of the oldest tour attractions in North America. Mammoth Cave National Park preserves the cave system, a part of the Green River Valley, and the rolling hills of south central Kentucky.
The Grand Canyon of northern Arizona in the United States, with an average depth of 1,600 metres (5,200 ft) and a volume of 4.17 trillion cubic metres (147 trillion cubic feet), is one of the world's largest canyons. It was among the 28 finalists of the New7Wonders of Nature worldwide poll.
Gorges du Verdon - The European Grand Canyon. One of the reasons the Gorges du Verdon is so popular is that it's accessible from both the North and the South. People come from all over to get a glimpse of the Gorges du Verdon's turquoise waters and interesting rock formations.
Exploring Sequoia's Crystal CaveTours include: Family Tour: 50 minute tour for all ages. Thirteen and Older Tour: 50 minute tour for ages 13 and up. Discovery Tour: 1 1/2 hours for ages 13 and up.
Hranice Abyss (Czech: Hranická propast) is the deepest flooded pit cave in the world. It is a karst sinkhole near the town of Hranice, Czech Republic. The greatest confirmed depth is 519.5 m (1,704 ft), of which 450 m (1,476 ft) is underwater.