The Ciliwung River flows from a volcano south of the Indonesian capital, through the heart of one of the world's most densely populated cities and almost into Jakarta Bay. Almost, because for the final mile or so of its course, the river would have to flow uphill to reach the bay.
The same is true for the rest of the half-dozen sewage-choked rivers that wind though central Jakarta. Unable to defy gravity, they've been redirected to canals that drain into the sea.
The reason these conduits are necessary is that Greater Jakarta, an agglomeration of 28 million people, sits on a swampy plain that has sunk 13 feet (4 meters) over the past three decades.