The human population is growing at an alarming rate. By 2050, there will be almost 10 billion people on the planet.
We've been hearing this for years, but where is all this growth happening?
Today, Tokyo is the most populous city in the world, with about 38 million residents.
New York City is still in the top 10 ranking, with its nearly 8.5 million people. But in the next three decades, that's going to change.
While population growth in the US and Europe stagnates, the number of people living in the Middle East, Asia, and Africa is going to skyrocket.
By 2030, these will be the new 10 biggest cities in the world, according to the United Nations' World Urbanization Prospects:
10. Mexico City, Mexico: 23.9 million people
People participate in a "Zombie Walk" at the Revolution Monument in Mexico City. (Alfredo Estrella/AFP/File)
9. Lagos, Nigeria: 24.2 million people
People crowd a street at the central business district in Nigeria's commercial capital Lagos ahead of Christmas on December 23, 2016. (REUTERS/Akintunde Akinleye)
8. Cairo, Egypt: 24.5 million people
Anti-government protesters take part in Friday prayers at Tahrir Square in Cairo on February 4, 2011. (REUTERS/Amr Abdallah Dalsh)
7. Karachi, Pakistan: 24.8 million people
Supporters of the political party Pakistan Tehreek-e-Insaf take part in a rally near the mausoleum of Mohammad Ali Jinnah in Karachi, Pakistan, on December 25, 2011. (Akhtar Soomro/REUTERS)
6. Dhaka, Bangladesh: 27.4 million people
People crowd on top of a train entering the Dhaka airport rail station September 24, 2015, as thousands of Bangladeshi Muslims head home to celebrate Eid-al-Adha. (Ashikur Rahman/REUTERS)
5. Beijing, China: 27.7 million people
Balloons are released as a crowd looks up during a military parade marking the 70th anniversary of the end of World War II, at Tiananmen Square in Beijing, China on September 3, 2015. (REUTERS/Rolex Dela Pena/Pool)
4. Mumbai, India: 27.8 million people
Devotees try to form a human pyramid to break a clay pot containing curd during the celebrations to mark the Hindu festival of Janmashtami in Mumbai, on August 10, 2012. (Danish Siddiqui/REUTERS)
3. Shanghai, China: 30.8 million people
Chinese national flags are seen as tourists crowd along the Nanjing Pedestrian Road, a main shopping area, on a national holiday in Shanghai on October 4, 2014. (REUTERS/Aly Song)
2. Delhi, India: 36.1 million people
Indian Prime Minister Narendra Modi performs yoga with others to mark the International Day of Yoga, in New Delhi, India, on June 21, 2015. (Adnan Abidi/REUTERS)
1. Tokyo, Japan: 37.2 million people
People hold placards and shout slogans as they gather to protest against Japan's Prime Minister Shinzo Abe's security bill outside the parliament in Tokyo on August 30, 2015. (REUTERS/Kyodo)
The Global Cities Institute projects that Kinshasa, in the Democratic Republic of the Congo, will be the most populous city in the world by 2075, and that by 2100, it will be surpassed by the 88 million future residents of Lagos, Nigeria.