The atmosphere inside the command compartment changed instantly.
Several crew members immediately looked toward the sonar station.
The operator increased sensitivity gain while another technician filtered background ocean noise.
A low sound echoed faintly through the compartment speakers afterward.
Deep.
Rhythmic.
Almost like a distant vibration moving through water.
Captain Weber stepped closer.
"Source?"
"Unknown, sir."
The operator continued adjusting the system.
The submarine's passive sonar array detected sound without actively broadcasting sonar pings. It allowed them to remain hidden while listening to surrounding waters.
And whatever this thing was—
It was loud enough to be heard even through passive systems.
Another sonar technician looked toward his display.
"Contact bearing two-seven-one."
"Distance?"
"Still calculating."
The operator swallowed slightly afterward.
"Movement pattern is irregular."
Captain Weber stared toward the display.
