Two thousand years ago, there were a mere 170 million people populating the entire planet Earth, according to WorldPopulationHistory.org. The human race has been on an upward trajectory ever since—logging in at 7.7 billion people in 2019 and projected to reach 10 billion by the 22nd century.

Our burgeoning population presents many challenges, according to the United Nations, especially as countries look to hit sustainability goals by 2030. Basically, the more humans there are on Earth, the more resources such as food, water, and the land we need, the more resources we take away from other species, and the more waste we generate.

There is some good news. Population increases are slowing down and in some countries, and elsewhere, some populations are shrinking, according to the U.N. Still, there are other places that are virtually bursting at the seams. For United Nations World Population Day, on July 11, we look at the 20 most populous metropolitan areas on the planet—defined as core cities plus their surrounding communities—as tallied by World Population Review.

20. Guangzhou, China

Current population: 12,967,862

With an almost 3 percent rise in population since 2018, Guangzhou, the southern Chinese city at the head of the Pearl River Delta that was formerly known as Canton, is one of the fastest growing cities around. It’s a commercial and trade hub—and has been since the 3rd century. Despite the physical expansion of the borders of the city over the decades and centuries, the vast majority of Guangzhou residents are packed into the city’s nearby districts.

19. Rio de Janeiro, Brazil

Current population: 13,374,275

Almost 5,400 people per square kilometer are packed into this massive city on Brazil’s coast. It’s one of the most popular travel destinations in South America, and yet over 20 percent of its residents live in impoverished conditions in almost 800 favelas, or so-called “substandard” districts around town, according to The Rio Times. On the plus side, though, population growth is slowing, down from a high of 4.2 percent in 1965, to .61 percent in 2019.

18. Tianjin

Current population: 13,396,402

The population of this 12,000-square-kilometer port city in Northeastern China expanded by a whopping 188 percent between the years of 1953 and 1982. Luckily, growth has slowed considerably since then—it expanded a mere 1.3 percent between 2018 and 2019—but it’s showing no signs of leveling off or even decreasing. Manufacturing is the largest industry in the city and accounts for its speedy and consistent growth.

17. Manila, Philippines

Current population: 13,698,889

Although growth in the urban population of Manila has actually slowed down over the years as the surrounding suburbs have seen an uptick in residents, that doesn’t mean the city is getting any less crowded. In fact, it has the “highest density [for a] major municipality in the world,” according to New Geography, averaging 14,500 people per square kilometer—with an astounding 70,000 per square kilometer in the city’s densest district. With many poor migrants moving into Manila from rural areas, Manila’s projected increasing growth rates are certain to put further strains on resources.

16. Lagos, Nigeria

Current population: 13,903,620

This port city—and the largest city in Africa—originated on the islands of the protected Lagos Lagoon, but has since expanded some 50-plus kilometers inland and now encompasses about 452 square kilometers. Its diverse population is represented by about 250 groups, many of which have emigrated both from the Nigerian countryside and from surrounding countries. The vast majority of residents—66 percent—live in slum conditions.

15. Kolkata, India

Current population: 14,755,186

India (1.3 billion) and China (1.4 billion) are the two most populous countries in the world, so it should come as no surprise that Indian and Chinese cities rank as some of the most crowded. Kolkata, a.k.a. Calcutta, a former East Indian Company trading post, as well as the headquarters of Mother Teresa’s Missionaries of Charity, is the third most inhabited metropolitan area in India—a trend that is slowing but, thanks to the increasing popularity of its suburbs, shows no signs of reversing any time soon.

14. Istanbul, Turkey

Current population: 14,967,667

Istanbul, formerly Constantinople, ranks as the most populated city in all of Europe, according to World Population Review (although technically, half of the city lies in Asia). This is a significant improvement of a sort, considering that it was the most populated city in the world back in 1502. Nevertheless, the city’s residents doubled their population in the 1980s, when the parameters of its borders greatly expanded. It boasts the largest airport in the world—a lucky happenstance for one of the fastest growing metropolises.

13. Buenos Aires, Argentia

Current population: 15,057,273

Although the capital city of Argentina has only 3 million people living within its urban core, its wider, 2,681-square-kilometer region is home to the second largest population in South America. Although the suburbs of the greater metropolitan region have burgeoned over the last 70 years, the city center has remained pretty constant, size-wise; its stagnant economy has meant that folks are more prone to move out of town than into it.

12. Chongqing, China

Current population: 15,354,067

Like many other cities in Asia, Chongqing has seen a big population spike—a hefty 3.5 percent since 2018. In fact, Chongqing is one of the fastest growing cities in the world and, as World Population Review points out, that’s not fixing to change anytime soon. The city has another problem in addition to its rapid expansion: Its workforce is shrinking while its population ages—an odd and sobering fact to consider in conjunction with the total population of the Chongqing municipality, which currently stands at a staggering 33 million people. By contrast, these are the smallest towns in America by population.

11. Karachi, Pakistan

Current population: 15,741,406

One-fifth of Pakistan’s urban population resides in Pakistan’s once-capital “megacity,” according to a report by the World Bank. While the number of its residents has soared—up 2.2 percent since 2018—its quality of life has been plummeting, with violent crime, inequality, and financial instability all contributing to developmental challenges. Karachi is also one of the world’s densest cities; it has 24,000 people living per square kilometer. Find out how most countries got their names.

10. Osaka, Japan

Current population: 19,222,665

The number of people living in Japan’s food-centric and third largest metropolitan area is actually decreasing—it’s dropped by .3 percent since 2018. But nearly 20,000 humans squeezed into the broader Kansai region (which includes the city of Osaka) of 27,000 kilometers makes for some tight living quarters. And in fact, 7 percent of Japan’s total population lives within an even smaller space in this region, in the city of Osaka proper and its 42 municipalities.

9. Beijing, China

Current population: 20,035,455

China’s second largest municipality has almost as many residents as the entire country of Australia. And it’s population has not only been rising at a steady clip since 1975; it’s projected to keep on rising all the way through 2035, according to the UN World Urbanization Prospects report. One of the serious downsides to such a large population in an urban area: Beijing has some of the worst air for breathing, thanks to nearby polluting coal plants and to car traffic—measuring 52 microns per cubic meter, as reported by Reuters. It’s also running out of water.

8. Mumbai, India

Current population: 20,185,064

India’s second largest metropolitan area has its largest in-city population, with over 13 million at last official census count in 2011, according to National Geographic. Its trains alone carry 6 million people a day, a figure that puts almost every other busy city to shame. Mumbai’s population has surged in the past 20 years—and has in fact doubled since 1991—as migrants from rural areas move to town looking for work. The practical realities of this flux are grim: reports World Population Review, 41 percent of residents in Mumbai and its surrounding regions live in slums.

7. Dhaka, Bangladesh

Current population: 20,283,552

Business Insider called Dhaka the most crowded city in the world in 2018, with over 23,000 people per square kilometer and an incredible 2,000 people per day moving into Bangladesh’s capital city. The reason for the influx: pure and simple desperation, as climate change-exasperated natural disasters wreak havoc on their home towns and villages. Not surprisingly then, one-quarter of Bangladeshis live below the poverty line, with an estimated 3 million people living in slums of Dhaka alone, says Business Insider.

6. Cairo, Egypt

Current population: 20,484,965

Egypt’s capital city has been settled at least since the 4th century due to its important location on the Nile River. After plagues in the Middle Ages and city-destroying riots in the 1950s, it’s since been growing at a rapid clip, according to ThoughtCo. Even its core city population is staggeringly high—12 million as tallied by World Population Review. Alexandria, the second-largest city in Egypt, is a mere 30 percent of Cairo’s size. By contrast, check out the smallest country in the world.

5. Mexico City, Mexico

Current population: 21,671,908

An economic powerhouse (it’s ranked 18th in the world for “strong economic clout,” according to WorldsCapitalCities.com), Mexico City is also one of the most populous and densest in the world. In fact, 20 percent of the entire population of Mexico lives in Mexico City, writes World Population Review. Unsurprisingly, surges in growth—an almost 3 percent hike since 2018—have meant that the city has had a hard time keeping up with necessary services for its residents, like housing.

4. Sao Paulo, Brazil

Current population: 21,846,507

The most populous city in the Americas, with over 11 million people living in the city itself, is a sophisticated, ethnically diverse world cultural center that is also, oddly, according to Forbes, home to 26 billionaires. It’s also a city of immigrants—”81% of students said they were descendants of foreign immigrants,” writes World Population Review of a recent survey by the University of Sao Paulo. Although the metropolitan area continues to grow—it’s projected to reach 23 million by 2030—it’s actually dropped in rank in the last few years.

3. Shanghai, China

Current population: 26,317,104

Shanghai is the largest city proper in the world, and one of the fastest developing for 20 years, seeing “double-digit growth nearly every year since 1992, with the exception of the global recession of 2008-2009,” says World Population Review. And it’s still growing, even though the pace of that growth is slowing. Experts predict its greater metropolitan regions will hit 50 million residents by 2050, which is sure to exacerbate all the problems that come with dense cities, like insufficient infrastructure and pollution. Shanghai is unlikely to make the United Nations’ list of “good” countries—those making a positive contribution to the world—anytime soon.

2. Delhi, India

Current population: 29,399,141

Delhi, India’s capital and the center of its government, is second in population and first in air pollution. It packs over 11,000 people into each of its 1484 square kilometers. And it’s growing faster than almost anyplace else on Earth. “In 2001 alone,” reports World Population Review, “its population increased by 215,000 due to natural growth and 285,000 through migration.” Its numbers increased this year by 3.25 percent over 2018 but it’s still on track to hit a population of over 43 million by 2035.

1.Tokyo, Japan

Current population: 37,435,191

Believe it or not, the number one most populated metropolitan area on the planet actually saw a population drop of .09 percent in 2019. That trend is expected to continue and to increase through 2035, so that 16 years from now, Tokyo will house almost 1.5 million fewer people than it does today. Needless to say, this is the kind of population trend that would benefit the rest of the cities on this list! See how America is catching up with the fastest growing cities in the U.S.