What's with the dead end in the top right corner of Tropos?
This is the passage that leads you to the pirate base camp area, which you visit in the main questline after Spiderman joined the team in Mesos. You were given the hint that there's a passage from Tropos to the pirate's base camp behind a waterfall, which is this waterfall.
Once you completed the pirate base camp stage, this passage is inaccessible because the pirate base camp is no longer accessible.