Predicted increase in population by 2025: 3.29 million
As the world's most densely populated city with over 46,000 people per square mile (nearly twice that of New York City), Manila is reaching its breaking point. Yet, the capital of the Philippines is still expected to grow by more than 3 million people by 2025. And with the predicted growth comes a host of challenges. Manila may be the Southeast Asian nation's business hub, but economic disparity plagues the city as spiking land prices make housing unaffordable for laborers who earn meager wages. As a result, the city has sprawled far outside comfortable reaches, making things like water distribution increasingly difficult. Currently the city relies on dated water pumps an unsustainable and dirty method of distribution that will need to be addressed in order for the city to sustain its growth. But Mother Nature could speak first: Manila is plagued by frequent earthquakes, a consequence of its location on the Ring of Fire, which coupled with its shoddy infrastructure makes the population boom increasingly precarious.