Underpopulation or low population density — often referring to depopulation in rural areas.
地方の過疎化が進み、学校が閉校になった。
Depopulation in rural areas progressed, and a school was closed.
過疎地域では公共交通が減少することが多い。
Public transport often decreases in depopulated regions.
若者の流出が過疎を加速させている。
The outflow of young people is accelerating depopulation.