Visualizing fall of empires

I guess fall of empire is fine example of a type of failure. I am pretty interested in learning the rise and demise of powerful empires.

Here is a nice video of visualizing fall of four maritime empires

more info here:

Living in an ancient city of Angkor, being surrounded by its remnants, I wonder how exactly it fell. 

Once upon a time, Khmer empire grew; empowered by its impressive water management systems which are still functional to date. The city now has problem controlling its sewage system.

Another empire I am interested in is Mongol empire which occupied half the world at its height.

There are a lot of things to learn from history. One thing I learn is the certainty of impermanence.