This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Contrary to popular belief, Genghis Khan did not conquer all the areas ultimately part of the Mongol Empire. At the time of his death, the Mongol Empire stretched from the Caspian Sea to the Sea of Japan. The empire's expansion continued for a generation or more after Genghis's death in 1227. Under Genghis's successor Ögedei Khan the speed of expansion reached its peak. Mongol armies pushed into Persia, finished off the Western Xia and the remnants of the Khwarezmids, and came into conflict with the imperial Song dynasty of China, starting a war that lasted until 1279 and that concluded with the Mongols gaining control of all of China. They also pushed further into Russia and eastern Europe.
What sea bordered Genghis Khan's empire to the east when he died?