NANTUCKET, Mass. -- A light fog was settling over Nantucket as a friend gave me a tour of the island late on a September afternoon. The road ended at a chain-link fence, its gate open, and she pulled over. ''It's the lighthouse,'' she said. ``I've been wanting to see this.''

Inside the gates was a strange sight that underscored the uneasy relationship between Nantucket and the sea.

The Sankaty Head Lighthouse, standing like an uncertain sentinel in the fog, was being moved. The 70-foot-tall lighthouse, built in 1850, sat on a bluff that over the years had been undermined by erosion. The dirt around the foundation had been excavated, and the tower had been lifted up onto steel I-beams.

The tower is just the most visible evidence of Nantucket's long battle with erosion. Some years, as much as 15 feet has washed away on the island's South Shore. Some homes along the Sankaty Bluff have been moved away from the edge; others have been destroyed.

A separate effort to rebuild the eroded Siasconset Beach has pitted worried homeowners against fishermen who say the work will ruin the fishing grounds.

A couple weeks later, the lighthouse was shifted to a concrete pad about 400 feet away. It has been saved, at least for now, but I wonder about the fate of all those houses overlooking the sea.

This is one in a series of postcards by Marjie Lambert, assistant Travel editor, who has been to all 50 states.