The Side-Scan Sonar

At one in the morning, the bay of Cartagena was an unbroken sheet of molten lead beneath a waning crescent that reflected upon the oily swell like a pale scar.

Leovigildo's skiff crawled at two knots with navigation lights extinguished to avoid alerting Coast Guard patrols from the naval base. In the bow, Soledad fed out the coaxial cable of a side-scan sonar transducer rented with her university savings.

The towfish slipped into the murky brine with a muted gurgle. Across the portable monitor mounted atop a beer crate, bathymetric lines traced across the screen in phosphor green.

—Hold the heading toward Santa Catalina bastion, Polo —Soledad whispered—. Don't slip off the drop-off contour.

—The helm fights me every time we near the yellow markers —the old man grunted, bracing bare feet against wet floorboards—. As though the undercurrent weren't water at all, but cold pitch.

Across the sonar display, the seafloor's chaotic silt topography began to organize into unnatural order.

At forty feet down, beneath the blanket of black mud, emerged a structure of terrifying geometric perfection:

These were no scattered stones tumbled from colonial curtain walls. They were eight concentric tiers of cyclopean masonry forming an octagonal amphitheater a thousand feet in diameter, centered squarely beneath the Bocachica anchorage. From each of the eight vertices radiated vaulted galleries penetrating the living bedrock toward the walled city.

—Holy Mother of Candelaria... —Leovigildo breathed, leaning over the screen with his cold cigar clamped between yellow teeth.

—It's no defensive breakwater —Soledad noted, dragging the digital calipers across the screen—. The walls don't face the open sea to break English warships. The embrasures and counterforts are oriented inward. Toward the center of the pit.

—Like an animal trap? —Polo muttered.

—Like a cage —she corrected him, an icy shiver racing down her spine—. A sunken cage engineered to hold something anchored to the floor of the trench.

Suddenly, the sonar receiver let out a high, saturated shriek. The monitor scrambled into a blizzard of white static.

At the center of the octagonal pit, one hundred and thirty feet beneath the mud where official marine charts indicated barren shale, the sonar registered an impossible displacement of mass: a sudden fifty-foot heave that rose and fell with the slow, terrifying rhythm of a colossal lung.

The entire skiff shuddered. The water encircling the hull began to boil with frigid bubbles that released a sickening stench of rotten limestone and sulfur.

Leovigildo yanked the diesel into full reverse, making the mounts groan while the tiller whipped in his grasp as though an unseen talon were wrenching the rudder shoe.

—Haul the cable, girl! —the old fisherman hollered—. Haul the line before it drags us down into the dark!