There is no definitive answer to this question. The phrase is a figure of speech and has been used to refer to different bodies of water at various times and places.

Some ancient civilizations used the phrase “seven seas” to describe the bodies of water known at that time. The ancient Romans called the lagoons separated from the open sea near Venice the septem maria or seven seas. Most current sources state that "seven seas" referred to the Indian Ocean, Black Sea, Caspian Sea, Adriatic Sea, Persian Gulf, Mediterranean Sea, and the Red Sea.

Not all geographers agree on this list of seven, believing that the seven seas reference will be different depending upon the part of the world and the time period in question.

Some geographers point to the Age of Discovery and suggest that the seven seas represent the Atlantic, Pacific, Arctic, and Indian Oceans, as well as the Mediterranean Sea, the Caribbean, and the Gulf of Mexico.

Other geographers state that the seven seas were the Mediterranean and Red Seas, Indian Ocean, Persian Gulf, China Sea, and the West and East African Seas.

Today we recognize more than 50 seas worldwide. A sea is defined as a division of the ocean which is enclosed or partially enclosed by land. With that said, the Caspian Sea, Dead Sea, and Aral Sea are actually saltwater lakes, because they lack an outlet to the ocean. Conversely, by this definition, the Gulf of Mexico and Hudson Bay are seas.

Interesting Sea Facts:

• The largest sea is the Bering Sea at 876,000 sq. miles or 2,270,000 sq. kilometers.
• The saltiest sea in the world is the Red Sea with 41 parts of salt per 1,000 parts of water.
• The warmest sea in the world is the Red Sea, where temperatures range from 68 degrees to 87.8 degrees F depending upon which part you measure.
• The coldest seas are found near the poles such as the Greenland, Barents, Beaufort, Kara, Laptev and East Siberian Seas found near the north pole and Weddell and Ross Seas found in the south poles. The Baltic Sea is also considered one of the coldest seas.
• Depending upon the amount of salt in the water, sea water freezes at about 28 degrees F. High salt content lowers the temperature for freezing and low salt content raises the temperature for freezing.