“For what?” Lena whispered.

The result made her coffee go cold.

Dr. Lena Marchetti first noticed the file because it had no owner. On the deep-sea research vessel Odysseus , every data stream—hydrothermal, biological, seismic—bore a scientist’s tag. But SAES-P-126 was a ghost: a continuous, low-frequency acoustic signature from the Puerto Rico Trench, recorded every 47 seconds for the past eleven years.

The signal changed. SAES-P-126 sped up. Pulses came every 4.7 seconds now. The ship’s sonar caught a hum that vibrated through the hull, through the crew’s molars, through the very marrow.