Although darkness could not pose any threat to Mu Lin, the icy seawater made him feel slightly endangered. He had been diving underwater for eight hours already, and the cold water had drained much of his body heat. Although his core energy, or Zhen Yuan Li, was barely consumed, his muscular energy had been heavily depleted, reaching the limit of his physical endurance—an outcome he had not anticipated before diving in.
He took out a large energy-replenishing pill from his ring and consumed it to restore his physical strength, then continued swimming through the ocean. Thanks to his adoption of internal breathing, his body could now directly extract oxygen from the seawater through his navel, much like a fetus in its mother’s womb. Thus, he no longer needed to surface for air and had become, for all intents and purposes, an aquatic creature freely roaming the ocean—an achievement that was the greatest reward of his underwater journey.
Since narrowly escaping death after being ambushed by the Y country’s government at sea, Mu Lin had been pondering a question: why can a human fetus breathe freely in the amniotic fluid without lungs, similar to aquatic animals, but after birth, this basic instinct is lost?
With this question in mind, Mu Lin decided to find an answer underwater. However, once submerged, he discovered through internal observation that the function of the navel was completely replaced by the lungs due to the balance between water pressure and the body’s internal pressure. The lungs and navel were like two water pumps in the body; once the lungs started working, the navel stopped.
After birth, humans live on land for prolonged periods, and the navel gradually ceases to function. If human embryos were not nurtured in a completely fluid environment, the navel—this secondary organ for respiration and nutrient absorption—might eventually become a redundant organ, much like the appendix.
The principle of “use it or lose it” is a law of nature. Although marine mammals like whales also breathe through lungs, Mu Lin speculated that perhaps the oxygen intake through the navel from water was insufficient to meet the needs of their massive bodies, forcing them to breathe air like humans.
Therefore, the navel must still function underwater—it was just a matter of how to activate it. With this idea in mind, Mu Lin first tried holding his breath underwater, but it did not work. After holding his breath for over twenty minutes, there was no sign of the navel awakening. He then experimented with various martial arts techniques he was familiar with, only to find that even so-called fetal breathing still relied on the lungs.
After some thought, he decided to minimize the use of his lungs underwater, hoping this might gradually reactivate the navel’s function. With this determination, he persisted in practicing fetal breathing underwater to stimulate the navel. After more than four hours, he finally felt his navel slowly awakening, while his lungs gradually reduced their workload. Slowly, his internal organs adapted to this change, though his body continued to lose heat rapidly.
At this point, Mu Lin finally understood why marine mammals store so much fat internally. Now, relying on his Zhen Yuan Li, oxygen absorbed through the navel, and energy-replenishing pills from his ring, he had truly transformed into an aquatic creature. From now on, in the ocean, Mu Lin would fear no one.
After swimming freely for a while, Mu Lin felt tired. He took out a small submersible from his ring, opened its hatch, and entered it before descending back to the beach. During the day, he rested in the wild, and at night, he resumed his underwater training. He had already reported his safety back home, so he had plenty of time to spare. Having solved the issue of breathing underwater, he now urgently wanted to tackle the second challenge: how to adapt his body to the pressure of seawater. Only by overcoming this challenge could he truly claim to have adapted to the ocean, making the sea and land indistinguishable to him. As for seeking revenge on Y country, that could wait.
The pressure underwater is different from that on land. The deeper one dives, the greater the pressure becomes. Generally speaking, every 100 meters of depth adds about ten atmospheres of pressure. Moreover, visibility underwater is extremely low, and the environment is harsh. The ocean floor is not flat. The world’s largest sea—the Coral Sea—has a maximum depth of 9,140 meters, making it the deepest among coastal seas. The deepest point in the world’s oceans is the Mariana Trench in the Pacific Ocean, reaching a depth of 11,521 meters.
Thus, unless one can personally explore the ocean floor, it cannot be said that one has conquered the sea. Relying solely on submersibles is inconvenient, and whether one can even borrow one is uncertain. Designing a manned submersible is a complex, multidisciplinary engineering task involving fluid dynamics, structure, materials, life support systems, hydraulics, underwater acoustics, optics, computer control, and more. In essence, manned submersibles represent the pinnacle of a nation’s technological advancement.
Currently, there are only five operational manned deep-sea submersibles worldwide: Japan’s “Deep Sea 6500,” the United States’ “Alvin,” France’s “Nautilus,” and Russia’s “Peace” and “Mister.” All of them have a maximum diving depth of only 6,500 meters—insufficient for Mu Lin’s ambition to conquer the ocean.
Mu Lin’s next task was to find ways to adapt his body to the deep-sea environment, aiming to reduce the limitations humans face underwater due to overreliance on lung breathing. These limitations include various conditions such as decompression sickness, nitrogen narcosis, carbon dioxide poisoning, and others.
To solve this, he needed to enhance his Zhen Yuan Li in the dantian to balance the external water pressure. Since Mu Lin now breathed through his navel underwater, he only needed to control his descent speed and gradually adapt to increasing depths to acclimate to the pressure.
The English Channel has an average depth of 60 meters, with its deepest point at 172 meters. The Dover Strait has an average depth of 30 meters, with its shallowest point at only 24 meters. Experiments have shown that without diving equipment, humans can dive no deeper than 20 meters. Therefore, Mu Lin began practicing gradually from shallow to deeper depths. Half a month later, he could already walk freely along the ocean floor.
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