John Block, age 11, of White Plains, N.Y., for his question:

WHAT CAUSES PERMAFROST?

Permafrost is ground that is permanently frozen. It is caused by a condition of the air. In regions where the average annual temperature is 32 degrees Fahrenheit or below, you'll most likely find permafrost.

Frozen ground may consist of dry rock, sand or soil. Some permafrost includes pure ice. Some is always covered with ice or snow.

In somewhat warmer regions, the permafrost may lie under a layer of earth that thaws in summer.

In some places, the ground is frozen to depths up to 3,000 feet. The bodies of such prehistoric animals as mammoths have been found preserved in permafrost.

Permafrost may thaw from the heat present in buildings and other structures. In summer, the thawed ground may become so muddy that foundations sink more than a foot.