Nina flicked her headlamp on and dove into the opening. The muddy floor was perfectly flat and about eight feet below the quay level. The light’s narrow beam picked out broken pottery and debris. Using her knife, she pried potsherds from the mud and put them into the collection bag after carefully marking their positions. She discovered a channel and followed it seaward until it broke out into the lagoon. The opening was easily big enough to allow for the passage of an ancient ship. The space cut into the quay had all the characteristics of an artificial harbor known as a cothon. She discovered several slipways, each big enough to accommodate ships more than fifty feet long, and a true piscine, which confirmed her theory about the cothon.

Leaving the quay, she continued on her baseline course using the land spit to her right as a reference point. She swam between the island and the mainland until she found a submerged mole or breakwater a few yards below the surface, constructed of paralleJ stone walls filled with rubble. In a drier time it would have connected the mainland and the island.

Coming to the island, she shed her dive gear and walked across thorncovered slabs of rock to the other side. The island was more than fifty feet wide, almost twice as long, and mostly flat. The trees she had seen from shore barely came up to her chin.

Near the lagoon entrance were piles of stones, probably foundations, and a circle of blocks. It was the perfect spot for a lighthouse or a watchtower, offering a sharpeyed sentinel a panoramic view of ship traffic. Defenders could be summoned from the mainland whenever a sail was sighted.

Stepping inside the circle, Nina climbed onto a fragmented stair and looked out at the anchored ship she had seen earlier. Again she wondered what would bring an American government vessel to this arid and lonely coast. After a moment she retrieved her scuba equipment. The cooling and weightless environment back in the water was refreshing, and she decided her fishy ancestors had made a big mistake when they crawled from the sea onto dry land.

Nina swam across the lagoon entrance. The other peninsula started low from the land, gradually widening as it rose to a knobby crag. The sheer reddish rocks dropped straight into the water like the ramparts of a fortress. Nina dove until she was at the base of the blank wall, looking for a footpath. Finding none, she continued underwater to the seaward end of the promontory which terminated in a rocky shelf. A. perfect defensive position where archers could set up a murderous cross fire to rake the decks of any invader entering the harbor.