Current population: 18.9 million
Any visitor to Indonesia's capital city can tell you it's a traffic nightmare. What was once a sleepy Dutch colonial outpost in the 17th century stands to be one of the most important (and congested) cities in Southeast Asia in the 21st century. Jakarta is a teeming metropolis drawing in migrants from every corner of the diverse archipelago nation, but poor urban planning and ramshackle development have led to annual floods that inundate much of the city, turning streets into rivers and daily life into a diluvial struggle. Tentative plans to tie the administration of Jakarta up with that of outlying areas surrounding it in West Java — creating a megacity with the awkward-looking name of Jabodetabekjur — are still infant and have yet to yield any real results for beleaguered commuters and city dwellers.