Answer: They were naturally defined by deserts, mountains, and seas.
Step-by-step explanation:
Metaphorically, four bodies of water were considered as the boundaries of Ancient China, deemed as the Four Seas (although two of them were only symbolic and later tied to lakes, not seas).
However, the real borders of Ancient China
were determined by natural geographical features, such as seas, mountains, the Gobu and Taklimakan deserts, the North China Plain, and rivers like Huang and Chang.