Complex sentence: Similar dolmens can be found outside of Korea, in Manchuria, Shandong, and Kyushu, yet it is unclear why this culture only flourished so extensively on the Korean peninsula compared to the area of Northeastern Asia.
Split into simpler sentences: Similar dolmens can be found outside of Korea, in Manchuria, Shandong, and Kyushu. Yet it is unclear why this culture only flourished so extensively on the Korean peninsula compared to the area of Northeastern Asia.