The first thing Ava heard upon waking from the anaesthesia fug was a long, low drone. It pricked up the hair on the back of her neck. Her arms came over with goosebumps. 

“D’you hear that?” she whispered.

Sidra’s still-goggled face swam into view. “Hear what?”

“I swear, Sidra—" she hissed, sitting up. "If you’ve given me a dodgy implant—“

Dark, powdery blood came away when she touched her earlobe.

“It’s fresh out the box!” Sidra flipped up her goggles; her mismatched eyes scrunched in sympathy. “Sometimes, these things take time to settle. When I had my eye done, I couldn’t see shit for two weeks. HUD says you’re good. It’s so small, it’ll be undetectable.”

#

Ava had to admit—Sidra knew her shit. When Ava retested for the squadron, she passed with flying colours. 

But that cryptic note in her head kept thrumming when she least expected it. 

Preparations for the dive were fierce—so many other vessels had lost signal, foraging the seabed’s silicon scrap. But when the drone came, Ava was useless. Once, she’d puked in a corner, then spent a whole double-shift hiding from the medics in a cupboard, heart juddering and skin filmy with sweat. They scribbled down ‘anxiety’ in her file, but she didn’t take the pills they prescribed.

Her first leave took her back to the Underbelly. Ava hammered on Sidra's door until the metal was barely dented and her knuckles went bloody and bruised.

“What’ve you done to me?” she rasped, when Sidra finally arrived.

“Nothing you didn’t pay me for.”

When she pulled up the serial number, Sidra hummed. “It’s a new-ish model. Maybe it’s picking up weird frequencies. I tuned it for your sonar-blips, but…” 

“It’s making me want to claw my skin off.”

The sound came again, then. For a while the world went white.

When Ava came to, Sidra was grey-faced. “Must be faulty. Come again, when you can. I’ll get it out.”

#

Ava went underwater before her next leave. 

She was in the control room when the drone came again. The whispering creaks in the hull made her skittish, but the captain was more concerned with protocol than paying attention to any deep-sea sounds. He buzzed: "Forward room, control, status update?"

His propriety was drowned out by the low thrum—unchanged, but somehow now almost sounding like singing. The captain's screen was a yawning void, but Ava finally understood. “There’s something below us—“ she said.

“Nothing on the sonar.”

“Believe me. There’s something there.” Something immense. Next to it, they were specks of dust.

“Officer, this is subordination!”

The long low note built between Ava’s ears. “Can’t you hear it?” she said, clutching at the captain's shirt. “Fuck, can’t you feel it? We’re prey. We’ve always been prey.”

All those missing submarines.

The radio gurgled to life. "Control, forward room, hearing something grinding along the hull?"

Their vessel shook as something rammed into the flimsy metal shell.

And the siren song became a scream.