According to geological records of Alaska, thousands of mysterious lakes of all the same shape have grown steadily for thousands of years. Amazingly, they are also the fastest growing lakes known in the world. While scientists have tried various ideas to explain the steady growth - the lakes expand up to 15 feet every year - and the lakes' consistent shape and orientation, but no theory has held up. Now a scientist who has worked previously on puzzles as wide-ranging as the spiral shape of Mars ice caps says he has solved the terrestrial mystery. The solution might also help explain a series of oddly similar lakes near the US East Coast. The lakes range from puddle-sized to more than 15 miles long. They are shaped like stretched-out eggs, with their skinny ends always pointing northwest. They are grouped in a vast field twice the size of Massachusetts. While lakes come in all sizes and shapes, they are rarely oriented in the same direction. The odd characteristics result from seasonal slumping of the banks when the permafrost thaws abruptly. Permafrost typically includes a top layer, called the active layer, that melts each summer and refreezes in winter. A lake grows when rapid warming from a sudden heat wave melts its frozen banks and the soggy soil loses strength and slides into the water. Previous theories had suggested, among other things, that prevailing winds were behind the shapes. However, according to the recent theory, heat waves are getting the credit. If the temperature warms gradually, the ice portion of the permafrost melts slowly, allowing the water to drain out of the soil and leave relatively firm sand or sediment behind. However, if an early heat wave melts the permafrost's ice rapidly, the result is a soggy, unstable soil. The lakes are all on a gently sloping landscape, and the downhill end of a lake always has a shorter bank. A computer model of the lakes revealed that shorter banks melt more and have bigger slumps. So when a lake experiences thaw slumping, the lake grows more in the downhill direction than it does uphill, generating the lakes' characteristic elongated-egg shape.