The waterfall

Jack headed up the path leading to the waterfall. Gladly there was nobody going there too and so he made it to the viewing platform without anyone seeing him. The platform was a bit above the waterfall pool and he felt the cool mist from the fall blowing into his face. He tried to remember a bit of his dream and then decided that he had to examine the wall behind the waterfall. There was a locked gate, although with it only going up to his hips, it was no real barrier for him. A small staircase led down to the pool, where the remainders of an old viewing platform lead around the pool.

The old platforms had gotten slippery and so he had to be careful as he walked around the pool and over a half-broken little bridge. Finally, he reached the stone wall and looked around. As he expected, there was no sign of any cave entrance. For a few moments, he felt rather silly, obviously, if there had been an entrance, it would've been discovered long ago by hikers or national park staff.

But then, he leaned against the wall and got wet from the waterfall dropping down just meters away from him, as the wall seemed to change for a moment. It was as if it lost its substance. A surprised sound escaped Jack's mouth as he felt through the wall, tumbling into the darkness behind.