1.Humans have climbed the world's highest mountains - but many underground caves are unexplored.
Caves are usually formed by water flowing through cracks underground. The water slowly dissolves certain types of rock, such as limestone. Over thousands of years, it can hollow out deep shafts, tunnels, and even huge underground chambers.
2.A cave in Malaysia contains the world's biggest single underground chamber.
The Sarawak chamber measures 600 meters long, 400 meters wide and at least 80 meters high. You could line up eight Boeing 747 aircraft, nose to tail, along with its length - and still, have space to spare.
V A view inside the Sarawak Chamber in Malaysia, the biggest cave chamber in the world. V
3.The world's longest cave system is Mammoth Cave in Kentucky, USA.
It has at least 570 kilometers of passageways - but explorers keep discovering more. Some experts think that only a small part of the whole cave has been explored.
V Cavers explore one of the many interconnected passageways in Mammoth Cave in Kentucky, U.S.A. V
4.In 2005, cave explorers broke a record, traveling more than 2 kilometers below the Earth's surface.
They were exploring the Voronya Cave in Georgia, near Russia. It is the deepest cave in the world, with a maximum depth of 2140 meters.
5.Workers in a mine in Mexico discovered an amazing cave by accident in 2000.
They were drilling a tunnel when they found a large chamber filled with enormous crystals. The crystals are made of the mineral gypsum. They look like giant swords, measuring 2 meters wide by up to 11 meters long. The chamber, which is around 300 meters deep, is now known as the Cave of Crystals.
V A geologist (a scientist who studies the Earth and its rocks) stands inside the amazing Cave of Crystals in Mexico. V
人类已经爬上了世界上最高的山脉，但许多地下洞穴还没有开发出来。
洞穴通常由流经地下裂缝的水形成。水慢慢溶解某些类型的岩石，如石灰石。几千年来，它可以挖出深井，隧道甚至巨大的地下室。
马来西亚的一个洞穴包含了世界上最大的单一地下室。
沙捞越小屋长600米，宽400米，高80米。您可以排队八架波音747飞机，鼻子尾巴以及其长度 - 仍然有空间。
V马来西亚沙捞越会议厅，世界上最大的洞穴室。 V
世界上最长的洞穴系统是美国肯塔基州的猛oth洞。
它有至少570公里的通道 - 但探险家不断发现更多。有专家认为，只有整个洞穴的一小部分已被探索。
V Cavers探索了位于美国肯塔基州Mammoth Cave的许多相互连接的通道之一
2005年，洞穴探险者打破了记录，在地球表面以下超过2公里。
他们正在俄罗斯附近的格鲁吉亚探索Voronya Cave。它是世界上最深的洞穴，最大深度达2140米。
墨西哥一个矿山的工作人员在2000年发现了一个惊人的洞穴。
当他们发现一个充满巨大水晶的大房间时，他们正在钻隧道。晶体由矿物石膏制成。他们看起来像巨剑，长2米，长达11米。这个大约三百米深的房间现在被称为水晶洞穴。
V地质学家（研究地球及其岩石的科学家）坐落在墨西哥令人惊叹的水晶洞穴内。 V