"CP 1765, October 15. 95 days have passed since landing. Local time: AD 2016, November 20."

  "It seems my imagination still falls short — or rather, I haven't fully grasped the power of religion first-hand. But at least I now understand why religion flourished here. It stems from the feudal era."

  "Specifically, I am referring to an age of swords, bows and armour. Here, the feudal era endured for an exceptionally long time. For over a thousand years, technology saw virtually no substantive advancement, while religions — nurtured by the very nature of feudalism — expanded to colossal scales. Dozens of nations, led by religious leaders, could wage religious wars, mobilising over 100,000 troops driven solely by differing faiths. And this happened more than once... religious wars like 'Jihad from such-and-such' and 'Holy War from such-and-such'..."

  "What astonishes me most isn't these feudal religious conflicts — after all, the prevalence of religion back then isn't entirely incomprehensible. But now, in the early era of space exploration, it still maintains such immense influence. Perhaps this is due to underdeveloped regions lacking scientific enlightenment, combined with the lingering effects of religion's former dominance?"

  "Another equally shocking fact is that most scientists of the Age of Enlightenment here were also theologians. I think I should better stop pondering this topic — otherwise, my life philosophy might turned upside down… Seriously, considering I've been on this alien planet for so long and my life philosophy is only now starting to waver, that's probably not too bad, right?"

  "Another curious thing is how follow-up research and progress on the nuclear saltwater rocket seem to be deliberately concealed. It's clearly a national measure to protect 'cutting-edge breakthroughs'. But why bother? They must realise that it offers almost no military utility, only significant value for future long-distance space exploration. Perhaps it's part of a space race — that's the only explanation I can find. However, since they have already released so much information, it seems pointless to cover it up now. Strange..."

  "As for adjusting my schedule, I doubt if I actually slept last night. This daytime-active, night-time-sleeping routine is utterly unnatural for our physiology, but the current pattern of alternating between daytime and night-time sleep is clearly even worse. It's a form of psychological torture. I hope this won't happen again..."

  "Weather-wise, it's snowing again. But the amount is pitifully small — barely enough to roll a decent snowball. Absurd. It's actually summer now... Indeed, the climate in this kind of place is better described as having a 'dry season' and a 'rainy season'..."

  ***

  "How's the new article coming along?"

  Caught off guard by Min's sudden question, Xi P feels as though he has been transported back years, to when his own professor would call on him unexpectedly. Regaining his composure, he replies: "Not good. I need to rewrite it."

  "What happened?"

  Xi P falls silent again. Min knows that this usually means he's gathering his thoughts, a flood of words will often follow soon after.

  But this time is different. Xi P turns and dashes back to study room, fetches his laptop, opens the abandoned draft, and displays it before Min.

  After listening to Xi P's explanation, Min also feels a headache coming on. The main reason isn't the complicated substitution problems, but that massive underground cavern. Nothing on Earth exists on such a scale, let alone one capable of supporting a biosphere. Driven by curiosity, Min asks a few more questions about the subterranean cavity.

  Xi P seems to possess an unusual passion for this subject. Despite the language barrier, he makes every effort to describe the place to Min. He even searches through his computer mid-explanation, successfully retrieving several photos and videos of the cavern.

  Ensure your favorite authors get the support they deserve. Read this novel on Royal Road.

  As Xi P continues his narrative, the contours of that subterranean cavern gradually become clearer in Min's mind. It all began with a massive earthquake beneath the capital city of a neighbouring nation. That quake not only triggered volcanic eruptions, nearly turning the capital into a ghost town, but its seismic waves also unveiled this earth-shattering scientific discovery—the Kaifenglia subterranean cavity, 6,000 metres below the surface. While mainstream theories attribute its formation to a strategically positioned earthquake, the exact process remains fiercely debated. This discovery has undoubtedly propelled significant advancements in geology and ecology. An ecosystem isolated from the surface for 50 million years is enough to drive ecologists wild with excitement.

  Xi P himself visited the site once. Admission was astronomically expensive - even more expensive than some astronomical journeys, though his trip was subsidised by the government. The reward? A truly breathtaking experience. In his own words, it felt "like stepping onto another planet". Yet if asked to choose between Earth and that subterranean cavern for the most alien sensation, Xi P would surely hesitate for a long time...

  If one must nitpick, 50 million years in geology and biological evolution is merely a medium-length span — roughly equivalent to Earth's Paleogene period. The reason species divergence far exceeds what would normally occur over 50 million years lies primarily in the unique environment and circumstances. Within such a confined, resource-scarce space, genetic advantages spread more rapidly. The evolutionary leap achieved in those 50 million years is likely to be comparable to what would occur over 80 million years elsewhere — a truly remarkable feat.

  At the very least, all species there had lost their eyes.

  A space only 30 cubic kilometres in volume could naturally support only a limited number of species. Only two species qualify as "producers", with one being a recent variant of the other. Both exist as blocky and filamentous structures in the soil, entirely dependent on bacteria in the adjacent soil and streams for nutrients. Though they are incapable of photosynthesis, these fungi function as producers within this unique biosphere, aided by a separate oxygen-producing cycle. Of course, this cavern contains numerous underground streams. Without them, this ecosystem, operating near collapse, could not persist for long.

  Animal species are slightly more diverse, with a full three types, all herbivorous. Competition between them is largely prevented by the widely separated streams, making conflicts rare. Even when clashes occur, such as over a stream, they pale in intensity compared to surface carnivores. Though eyeless, their appearance differs vastly from the Aliens (movie) and they cannot observe their surroundings via infrared. Simply put, they resemble small spheres with four stubby legs, attacking by biting with broad, blunt mouths. This often results in stalemates where neither side can break through the other's defence. Victory is often determined more by sensing the opponent's will to fight or level of exhaustion. Considering the scarcity of food resources there, this method isn't entirely absurd...

  This description makes Min laugh out loud, and when he watches one of the videos, he just can't stop. Xi P understands: the combat is indeed comical, especially that moment of mid-air collision, almost resembles some kind of ball sport... In fact, compared to the prolonged biting that follows, the initial mid-air impact probably inflicted far more damage. Most bites barely break the skin, while such collisions can actually damage internal organs.

  Though everything there can deliver immense shock to first-time visitors, boredom sets in quickly upon prolonged exposure. It isn't just the natural claustrophobia from prolonged underground confinement; most areas there are just boring, consist of barren plains with only natural stone pillars. Only near streams could one occasionally spot a few creatures — pale and grotesque in form. And that's essentially all there is to it. Even stepping out to explore on foot requires full protective gear, as oxygen levels are dangerously low. Compounded by the suit's poor cooling system, Xi P could barely endure more than ten minutes before retreating back to the rover. There really isn't much to see anyway.

  "Honestly, that place feels more like an alien planet."

  "I can't agree more."

  After Xi P finally finishes speaking, silence descends upon the living room, as if infected by the atmosphere of the subterranean cavern. It is only broken when the silver fox suddenly climbs onto Xi P's lap.

  "Are there any similar places on Earth?" Once Xi P finally shakes off the pale and grotesque alien creature, he refocuses on the conversation and asks.

  'Didn't you say you were rewriting it?' flashes through Min's mind. Still, he brainstorms for a moment and then tells Xi P about the mangrove forests. While mangroves aren't nearly as unique as the Kaifenglia subterranean cavity, they're the most distinctive ecosystem Min can think of. Perhaps deserts, limestone caves, or the ocean floor would be more interesting, but places like those surely exist in Xi P's world too...