In Xixia, "Han Hai" refers to the swamp south of Lingzhou (now southwest of Lingwu, Ningxia). (See History of Xixia)
The word "vast sea" began to refer to "desert" in Yuan Dynasty, and now the Gurbantonggut Desert in Xinjiang has been called "vast sea". (See Lu Ye's "Chu Cai Journey to the West")
After the Ming Dynasty, "the vast sea" specifically refers to the Gobi Desert.
It can be seen that the "vast sea" originally refers to the "sea", that is, the great lakes in the north, and later refers to swamps, vast areas in the north and the Gobi Desert.
The change of its significance may be due to the abundant rainfall and numerous lakes in ancient North China. Later, due to climate change, rainfall decreased, and the lakes in the northern Gobi desert gradually dried up, disappeared and even became deserts. This has led to a change in the meaning of the word "vast sea", which originally meant the Great Lakes in the north.
Reference: Baidu Encyclopedia