Why cant japans cities expand farther

Japan is experiencing a "super-aging" society both in rural and urban areas. ... The dramatic aging of Japanese society as a result of sub-replacement fertility rates and high life expectancy is expected to continue. Japan's population began to decline in 2011.The government will have to spend more on healthcare, and that, coupled with a shrinking workforce and tax base, is a recipe for economic stagnation. It also means, among other things, that there will not be enough young people to care for the elderly

Explanation:

answer ( groundwater);