In 3.15 Expedition league, a new mechanic was added: Logbooks. Underground areas can be blown up and accessed using the explosives. A new fishing-related are was found: Alluring Pool.
Currently the most valuable examples are encounters in which the Fisherman "chest" drops a normal Fishing Rod. Out of dozens of examples tested at the time of this writing, we only have n=2 examples in which a normal Fishing Rod drop has occurred.
We do not yet know if these drops are "Pure RNG" or if something more interesting might be going on. (This matters because if the rod drops are not "Pure RNG", the special requirements might give us a clue or new information about other fishing secrets.)
The current understanding and best chance of forcing a spawn of an Alluring Pool (as of 2/22/22) is by following these steps:
Logbooks that match all these conditions can be rare. Consider splitting ones you find, so that you have two attempts instead of one.