Japan has a very interesting geography. On one hand you have massive mountains that are very steep, and suddenly you drop to these extremely-flat plains that are close to sea-level.

On the central part of Honshu island (the biggest Japanese island), there is the Kansai plain, the largest city here is Osaka. Towards the east, you can see the Kanto plain which is also home to a massive population, and the largest city in the world by metropolitan population -- Tokyo (pop. 37,274,000).

The vast majority of Japan's 100+ million people live in these relatively small and flat plains.