The mountains around Guilin are limestone, and the constant movement of water through the rocks has led to a vast network of sinkholes, caves and underground tunnels. Here are two of them, hidden behind the foliage. You can see one quite clearly while the second one is lower down amidst the brush. Credit – shankar s. from Dubai, united arab emirates. CC SA 2.0. At the bottom of a 630-foot sinkhole in China lies a green surprise: an ancient forest with trees stretching nearly 130 feet (40 meters) out of the depths. Scientists believe it could contain undiscovered plant and…