What does it mean to come back to Wulongtan after a long time?

Going back to Wulongtan for a long time is a long time. Original: Peeping into the dark, throwing pebbles, the cave is full of water, and its sound is fierce and fierce for a long time. Look into the hole, it's dark. Throw a pebble in and there will be a loud sound of running water, which lasts for a long time.