Patti Torbeck, age 14, of Baton Rouge, La., for her question:

HOW LONG IS THE GREAT BARRIER REEF?

The Great Barrier Reef is the largest known deposit of coral in the world. The reef is in the Coral Sea off the eastern coast of Queensland, Australia. It extends in a northwest direction more than 1,200 miles long from a point opposite Mackay, Queensland, into the Torres Strait, which separates Australia from the island of New Guinea.

The channel between the Australian coast and the reef varies in width from about 10 miles at the northern extremity to about 150 miles in the south.

The channel, which is generally shallow and is dotted with islets and atolls, is extremely hazardous to navigate, particularly at the narrow northern extremity.

The reef itself serves as a barrier to disturbances in the Coral sea, thus affording a sheltered passage for ships.

In many places the reef protrudes above the surface of the water only at low tide. At high tide it is often indicated only by the turbulent state of the water.