To enter the cave, you must pay a levy of two fishes to the bear.
If you have not brought the necessary remittance, fret not. (Though I must make sure the old guard who permitted you to enter these woods reminds every visitor of the rules, provided he remembers them in the first place.)
If you carry down this trail, you will reach its end where it simply drops into the rapids. This is a good place to go fishing even if you have brought no gear with you. Simply dip a hand in the water and every fish, big and small, will scuttle to inspect the oddity, so few people have walked this way in centuries, giving you ample opportunity to grab one or more.
Pay no attention to the signs that warn you to steer clear of the water lest you wake the monsters that slumber under the riverbed. They too went to sleep centuries ago when the humans forgot about this place.
When you catch the fish, please don’t kill them. Bring them to the bear alive.
And if all of this does not sound like your idea of an adventure, simply go back the way you came and return, at your convenience, with two fishes. It doesn’t matter what kind so long as they are alive.
You could also offer your once-upon-a-child’s bath toy fishes. Children’s things throb with everlasting life even if it is not always obvious.
The bear isn’t terribly discerning, although he knows what can go into his mouth and what shouldn’t, much better than humans do for that matter.
Once you’ve paid the price, he will invite you to enter his cave. And you will find other forgotten lands to lose yourself in.