To fourfold something is to multiply it by four. When you fourfold a number, you get the number multiplied by 4 times the original number.
Slums are neighborhoods that are usually very poor. They are usually located in cities, and they usually have a high rate of crime.
A megacity is a city that has a population of over 10 million people. There are currently only a few megacities in the world, and they are all in Asia.
Ungrounded is a word that is used to describe something that is not grounded. This means that the thing is not connected to anything else, or it doesn't have any real weight.
An apocalyptic event is a very bad thing. It can be anything from a natural disaster to a war. When people talk about an apocalyptic event, they are usually talking about a very bad thing that is going to happen.
Overpopulation is when there are too many people in a particular area, and it is causing problems. There is not enough space to live, and the resources that are available are not being used efficiently.
To flourish is to grow and happen in a big way. When a plant flourishes, it grows tall and strong.
The billionth is a number that is very large. It's equal to 1,000,000,000. This number is so large that it's impossible to say it with words.
When something doubles or multiplies, it's called doubling or multiplying. But when something increases by a hundredfold, it's called tenfold.