Earth's middle sea is the Mediterranean Sea. You know, because it's surrounded by land. It's a sea in the middle of all that earth.

Actually, the Japanese name 地中海 matches the Latin roots of the English name too: medius for "middle" and terra for "earth." Whoa, did we just teach you some Latin? I sense a new business venture here. Wanius Kanius, anyone?

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.