Seoul

"Seoul" means "capital" in Korean. With a population of more than 10 million, it’s one of the largest cities in the world. Despite opposition, the Korean government has
confirmed plans to relocate the seat of government to Yongi-Kongju 100 miles to the south.

The government cites the heavy concentration of population as the main reason for the move:

Forty-six percent of Korea's population is concentrated in the Seoul Metropolitan Area. Compared with the population of other countries - Japan, 32%; Great Britain, 12%; France, 19% - Korea's population concentration is by far the severest and indicates the urgency of addressing the issue of population concentration in the Seoul Metropolitan Area. This heavy concentration has been a great barrier to achieving balanced regional development and national reconciliation by widening the gap between the Seoul Metropolitan Area and other regions. In addition, the Seoul Metropolitan Area is faced with many socio-economic problems, such as high housing costs and land prices, traffic congestion, environmental pollution.

Update: More information on the move in today's Guardian, which also has some good links to other sites about Korea. I still can't find a proper map, though. There's a very general one at the foot of this government page.