Answer:
Sea level
Explanation:
In geography, 'Elevation' is measured in relation to sea level, so sea level would be zero.
Thus, Chicago is 600 feet above sea level and Seely, California is 200 feet below sea level.
If there were nothing but flat land all the way from Seely to the ocean, sea water would come in and cover Seely. Fortunately for the people in Seely. California's coastal mountain range is between the town and the ocean, so that does not happen.