Until recently, Koreans were thought to have invented under-floor heating, a system they call "ondol". It was first thought to have been invented by the people of the Northern Okjeo around 2,500 years ago. However, the recent discovery of a c. 3,000-year-old equivalent indoor heating system in Alaska has called current explanation into question. The absence of prehistoric and/or ancient ondol features in the area between the two archaeological sites makes it unlikely that the two systems might have come from the same source. However, there has also been hypothesis that whale-hunting people from the Korean peninsula have migrated to Alaska by sea during the time period, and this could explain the phenomenon.