With no dive buddy to check her equipment, Nina went through the routine predive inspection twice. Satisfied with the results, she sat on the stair and worked her feet into her fins, then she slipped off the step before the blistering North African sun cooked her inside the wetsuit. The tepid water seeped between her skin and the neoprene wetsuit and quickly warmed to body temperature. She tested her main and extra regulators, then pushed away from the stairs, turned, and slowly breaststroked into the pondlike lagoon.

There was virtually no wave motion, and the slimy water was slightly brackish, but even with the surface scum Nina reveled in her freedom. She glided along with gentle fin flutters, pitying the expedition’s land archaeologists as they crawled on sore knees wielding trowels and whisk brooms, eyes stinging with sweat-caked dust. Nina could maneuver in comfortable coolness like a plane making an aerial survey

A lowlying island topped by an anorexic scraggle of stunted pines guarded the entrance. She planned to swim directly toward the island and bisect the lagoon: She would explore each half separately, making a series of parallel runs at. right angles to the baseline. The search pattern was similar to that used to find a wreck in the open ocean. Her eyes would take the place of a sidescan sonar or magnetometer. Precision measurements came later. She simply wanted to get a feel for what lay underwater.

Once below the clouded surface, the water was relatively clear, and Nina could see to the bottom, a depth of no more than twenty feet. This meant she could snorkel and conserve air. A series of intersecting straight lines materialized and formed into rectangles created by carefully fitted stone blocks.

The stairway had continued down underwater to an old quay. It was a significant discovery because it indicated the lagoon was once a real port and not a temporary anchorage. The bottom was likely to be covered with layers of civilization over a long period of time instead of junk tossed over the side by transient sailors.

Soon she picked out thicker lines and piles of rubble. Building ruins. Bingo! Storage sheds, housing, or headquarters for a dock and harbormaster. Definitely not an overnight anchorage.

Darkness loomed, and she thought she was at the end of the quay. She passed over a large square opening and wondered if it could be a fish tank, what the ancients, called a piscine. Far too big. The size of an Olympic swimming pool.

Nina spit out the snorkel, bit down on the regulator mouthpiece, and dove straight down. She moved along one side of the yawning cavity. Coming to a corner, she turned and followed another edge, swimming until she had covered the entire perimeter. It was around one hundred by one hundred fifty feet.