The term 'BIG EXPANSE' can be broken down into two crossword solutions: SEA (3 letters) and BIG (4 letters). The term 'BIG EXPANSE' is often used to describe a large area of land or water, such as an ocean or desert. The solution SEA is a reference to the vast expanse of water that covers most of the Earth's surface. The solution BIG is a reference to the size of the expanse, which is usually much larger than the surrounding area.