Submarine cables are used when electricity is transmitted across a strait, or to an island. Today, SCFF cables are not the mainstream in underground transmission cables. In dealing with high transmission voltages, however, they are superior to XLPE cables because an SCFF cable several kilometers long can be manufactured. Because of this characteristic, these cables are standard use for submarine cables. Note that manufacturing submarine cables capable of transmitting electricity stably for a long period under the harsh natural conditions at the bottom of the sea requires extremely advanced technology. Optical fiber composite cables have been increasingly used.