A certain year, a certain month, a certain day.

A certain sea area.

200 meters below the surface of the ocean.

Dusk Zone.

Underwater at this depth, the sun is as dark as dusk.

Looking down, there is a pitch-black abyss below.

Looking up, it looks like a brief period of dark blue sky after the sun has set and before it gets completely dark.

Unlike the real sky, there are no tired birds here, and occasionally a streamlined shadow swims quickly overhead, a large shark in the shallow sea.

This depth is inaccessible to at least half of the sea creatures, but there is still something creeping in the dim light.

Whew-

A shadow swam by at breakneck speed, its body reflecting a dim silver light, and few things could cruise at such speed in the sea.

It was an adult swordfish with a body length of nearly two meters, and it swam wildly with a long jaw shaped like a fencing, and in this deep ocean, it had almost no natural predators except for sharks due to its extraordinary speed.

Its eyes are large, the size of a tennis ball, and there are a large number of photoreceptor cells distributed in it, and it is impossible to hunt in this level of light without some peculiarities in its body.

With a sharp long jaw, keen vision, a body weighing hundreds of pounds, and a short-distance sprint speed of more than 100 kilometers, it has gained proud capital, and it is as terrible as a harpoon, and even has the reputation of "wearing a plank every day......

This swordfish is unlucky today, he has not found anything to eat, and hunger has made him a grumpy old man, eager to find something to get in the way of the day, whatever is fine.

In front of its high-speed swimming route, a big black creature suddenly appeared, and the slow swimming speed was a whale seven or eight times out of ten.

Even the fastest whale can't eat the tail of a swordfish, so it's confident that it can provoke it.

It may be that it is so fast that it rushes over without even seeing what the other party is, and when it gets closer, it realizes that something is wrong, and hurriedly twists its strong muscles to make a sharp turn, which can avoid the collision with the other party.

Why is this guy's body so long? much longer than the largest whale he's ever seen, with a round head, a "dorsal fin" as thick as a reef, a pectoral fin so small that it's almost invisible, and a tail fin that periodically rotates and stirs the water, and it doesn't look like a clumsy whale...... The most important thing is that the other party's skin seems to be much more difficult than the plank.

Forget it, let's go find a soft persimmon and pinch it.

With a flick of its tail, it swam away and disappeared into the dim light.

Mistaken for a whale by a swordfish, the nuclear submarine cruised silently underwater at a speed of 6 knots, passing under dozens of different types of vessels along the way, including fishing boats, freighters, tankers, private yachts, and even ventured into close encounters with warships, but fortunately was not spotted by any of the vessels.

Behind it is a long "tail", longer than any tail in nature, with a total length of almost 1 kilometer, including a towing cable 800 meters long and less than 1 centimeter in diameter and a sonar array of more than 100 meters long consisting of thousands of hydrophones.

The towed sonar array operates at wavelengths below 3,000 Hz, with a minimum effective frequency band of 10 to 20 Hz at very low frequencies, and can reach a maximum detection distance of 180 kilometers through deep-sea channels and convergence zone effects, and can even obtain intelligence and topographic features beyond the territorial sea boundaries of other countries.

Narrow sonar room.

Sonar 1, dressed in military uniform and wearing headphones, stares intently at the screen in front of him, looking for traces of enemy nuclear submarines.

Everything coming from the 180-kilometer radius is captured by the sonar array, amplified by the output, and then fed to Sonar 1's headphones, where a tuned indicator displays a waveform on a screen in front of his eyes.

Sonar No. 2, who sat next to him, was responsible for the operation of the bow sonar and echo sonde.

Sonar No. 1 periodically switches buttons and adjusts the filter, sometimes gaining low frequencies and sometimes high frequencies in case an enemy submarine is quietly approaching in the shadow zone.

The stubble on his lips proved that he was young, but he had been on duty in this submarine for a year, and he was not a rookie who panicked when something happened, as could be seen from his calm and sophisticated expression.

He knew most of them well, and if he hadn't done that, he wouldn't have been qualified to get off the boat and become the eyes and ears of the submarine.

The nuclear submarine is very quiet, makes very little noise on its own, and is far less noisy during operation than a diesel submarine.

He could distinguish the crisp crackling sound of shrimp bouncing its body, the sharp chirping of dolphins, the unpleasant sound of the golden drum fish, and the cooing of the big yellow croaker like a frog...... This kind of acute insight, which is almost incredible in the imagination of others, is only a part of his work and training for him, and he does not have unusual hearing, but he hears a lot and remembers it—without him, only familiar to him.

Suddenly, his straight eyebrows moved slightly, and an unfamiliar voice was heard in his ears, which was fleeting.

He didn't miss a single anomaly, the waveform on the screen just now still remained in his memory, adjusting the button according to his intuition, turning on the low-pass filter, and gaining low frequencies.

The voice returned.

His eyes rolled quickly, and he glanced at the frequency slightly, and saw that the range of the sound was between 45 and 50 hertz.

Is it a whale?

The submarine often encounters whales, and he is familiar with the sounds of various whales, and some whales can indeed make sounds in this frequency range in his memory, but the specific calls are different from what he just heard.

"No. 2, please confirm the bearing, band 45-50. He said.

Since the towed sonar is a straight line, it is difficult to confirm whether the source of the sound is port or starboard, and other sonar assistance is required, or it can be confirmed by the submarine's maneuvering change of direction, but a submarine in a cruising state cannot change direction just because of a slight anomaly, so the former can only be chosen.

"Understood. Sonar No. 2 replied crisply, turning the bow passive sonar to a low-frequency gain of 45 to 50 Hz.

The same young sonar No. 2 stared and listened for a while, and said, "Bearing 090, unidentified. ”

"Roger!" said sonar No. 1.

"Sir!" he took off his headset and called to the officer on duty.

"What's the matter?"

The officer on duty immediately walked briskly, and the sound was heard. He was much older than the No. 1 sonar officer, and he was truly mature and sophisticated, as evidenced by the wrinkles at the corners of his eyes and the rank on his shoulders.

"Suspicious contact, bearing 090, sounds like a whale. Sonar No. 1 reported succinctly.