I posted this in another thread, but I figure a post like this is more apt for this thread. So I'm reposting it here.
So where are the SH's headed to after Eneas Lobby? Is it Impel Down, Marine HQ, W7, or perhaps somewhere else? Personally, I'd rather see the Strawhats heading back to W7. The W7/EL arc has been a really long arc so far. We definitely need a resting period after this arc is over. And we're not going to get that if the Strawhats are traveling within enemy territory straight into the heart of the enemy. However, returning to W7 appears to be impossible atm. There's the problem with crossing the "waterfall of death." Then there's the problem with navigating through the aqua laguna. Both of these obstacles appear seemingly insurmountable.
So how can the Strawhats return to W7 with these two obstacles in the way? First they'd have to cross the "waterfall of death." To do this, at first I pictured the Strawhats using some sort of flying machine, like a dirigible or a helicopter, to cross the waterfall. And the presence of flying machines in Eneas Lobby could perhaps be explained by the fact that they're used to cross the calm belt. But the more I thought about this the more I realized how implausible this would be. If they marines had something like flying machines, then why don't we see more of them in the OP world? Clearly there are military advantages to using them.
Then I thought of the waterfall itself. If an obstacle is blocking your path why not get rid of the obstacle? Why not get rid of the waterfall by filling it? Ok, this is not that implausible a theory as it would first appear to be. Bear with me.
For all the natural phenomena we've encountered so far in the OP world, there have always been natural explanations for them (e.g. the knock-up stream, the reverse mountain currents, etc...). But no explanation has been offered yet for the waterfall. Why?
My theory is that there is no natural explanation for the waterfall. And if the waterfal isn't natural, then it has to be artificial. Now why would the WG create an artificial waterfall? What benefit would it serve? The answer lies with the calm belt, and the problem of traveling the calm belt.
How can you travel the calm belt? The solution: the calm belt can be traveled if you can create a current. And how do you create a current? Easy (well maybe not so easy), create a waterfall. Now navigating the calm belt is simply a matter of traveling on this current towards the waterfall (think of a conveyer belt. It's analogous to that).
There is only one problem with this theory however. The current created would only be a one way current. This explains how the marines are able to travel to EL from Impel Down or World HQ, but not vice-versa. So how are the marines able to travel from EL to either of the two destinations? Easy. By creating more artificial waterfalls, one near Impel Down and one near World HQ so that you create "conveyer belts" from EL to the two destinations.
So this explains why the WG would want to create an artificial waterfall. So how do you destroy this artificial waterfall? Well, if it can be created by man, then most likely it can be destroyed. Just destroy whatever contraption is responsible for creating the waterfall.
And that solves the "waterfall of death" problem. What about the aqua laguna? Well, the aqua laguna is probalby also caused by the artificial waterfall. All that falling water eventually has to be pumped out to somewhere. And it's probably being pumped towards W7. This is probably the main cause behind the aqua laguna. So getting rid of the artificial waterfall also solves the aqua laguna problem.
Also since the artificial waterfall is responsible for the marines' ability to cross the calm belt, destroying the waterfall also limits the marines ability to pursue the Strawhats while they make their escape. And it may also be instrumental in stopping the buster call - but hopefully just partially, since I'm hoping to see Usopp take on the buster call.