补全空缺短句，奖励100HY^^[cloze] For a growing band of intrepid young Chinese explorers, caves represent the ultimate adventure.__1__, a journey which endless raindrops will have followed over countless centuries. Fed by countless drips and trickles, the subterranean river carves ever deeper into the rock.

The cave river’s course is channeled by the beds of limestone. A weakness in the rock can allow the river to increase its gradient to flow in,__2__. The downward rushes halted when the water table is reached. Here the slow-flowing river carves tunnels with a more rounded profile. This tranquil world is home to specialized cave fishes, like the eyeless Golden Barb. China may have more unique kinds of cave-evolved fishes than anywhere else on earth. Above the water table, ancient caverns abandoned by the rivers slowly fill up with stalactites and stalagmites. Stalactites form as trickling water deposits its tiny quantities of rock over hundreds or thousands of years. Stalagmites grow up where lime laid and drips hit the cave floor.

So far, only a fraction of China’s caves__3__. And cavers are constantly discovering new subterranean marvels, many of which are subsequently developed into commercial show caves.

Finally, __4__, the cave river and its human explorers emerge in a valley far from where their journey began. __5__ the adventure is over. [/cloze]