英语听力：冰河绿洲—15

Could it be that drastic water shortage was the reason for the mastodons' epicmigrations? Another big piece of this puzzle lies far out to sea.

This is the ocean floor. But this isn't rock sprouting out at the bottom. It's wood. It's theremains of prehistoric tree stumps, some dated at more than 12,000 years old.

This sunken forest is unmistakable evidence that what is now seabed was once dry ground. And what is now Florida's coastline was once many miles in land. But why? To answer that, we have to go back to the mighty ice age glaciers that covered almost half the continent.

These glaciers contained immeasurable amounts of ice. So much water was locked up in this ice that it lowered sea levels by over 70 meters. The southeast coastal shelf was exposed and Florida doubled in size. The Everglades were dry land.