Before you go around asking 'Oh, whatdahell is this perrenial' or 'hey where can I get some of this' just visit the little known site of Google.com and do a search.I have just began working on a mod for Badspot's 'Perrenial' which adds a few more levels to the current prototype, and pretty much wraps up where you left off (in the ocean >.>)
Anyways, once you obtain the ground pound and mario-type block-breaking capabilities, you have the option to go back to the beginning of the woods, which yields an entrance to the underground city- a civilization long gone. Navigating the city, you must plunge into a hellish world filled with demons (a.k.a. insects) and monsters (a.k.a. crabs and such), and find your way back to the surface.
Advantages: Gives a dangerous-but-fun adventure through hell, but serves as a shortcut to the temporary end of the game.
This is currently in development. If you want to at least experience what Perrenial is like, check it out on Badspot's website. I have no current social media to track my progress.
Cool screenshots:
--- Jumping from building to building in Underground City ---

--- Same exact story with this one ---

--- A nice trip to hell (Incomplete)/Jumping feet-first into hell ---

--- .... And my annoying-ass modif. of the title screen, completed with blue link's lowres megusta faec ---

Have a good one, Kids!