Robert Serocki, age 12, of Solana Beach, Calif., for his question:

HOW DEEP IS THE DEEPEST CAVE?

The most extensive cave system in the world is found at Mammoth Cave National Park, Kentucky. It has 181.44 miles of underground passageways.

But the deepest cave in the world is Gouffre de la Pierre Saint Martin, located in the Pyrenees between France and Spain. This cave is 4,370 feet deep below its entrance.

Going down 4,258 feet is Gouffre Jean Bernard in France. Winning third place honors is Kievskaya in Russia, which is 3,543 feet deep.

The world's largest ice caves are found at Werfen, Austria. Called Eisriesenwelt, the caves have a length of 26.1 miles.

The world's tallest known stalagmite is in Aven Armand cave in France. It has attained a height of 98 feet from the cave's floor.