Why do certain lunar craters spark speculation about artificial structures beneath them?

Sure, no problem. The question about why some lunar craters spark speculation like "there's a base underneath" is indeed a fascinating topic. Let's discuss this in plain language.

Hey, that's a really interesting question and definitely one of the enduring mysteries surrounding the Moon. When many people see photos of the Moon, especially certain specific craters, they can't help but wonder, "This thing... looks a bit off, doesn't it?" This kind of speculation isn't entirely pulled out of thin air; it stems from some genuinely observed phenomena that are hard to explain using our everyday experience.

Let me break it down for you. There are a few key points that make some craters appear "anomalous":

1. Disproportionately Shallow Depth

What do we see? We observe some lunar craters that are incredibly wide, spanning hundreds of kilometers, yet surprisingly shallow. Logically, an impact powerful enough to create a crater hundreds of kilometers across should also have blasted a very, very deep hole.
Why does this spark imagination? Think about it: a giant rock hurtling at high speed into a sandpit would create a large, deep, bowl-shaped crater. But these large lunar craters look more like giant plates than bowls. This leads some to speculate: Did the impactor hit an incredibly hard, impenetrable layer at a certain depth? What is this "ceiling"? Could it be some kind of artificial "shell"?
The scientific explanation? Geologists attribute this to a phenomenon called isostatic rebound. Imagine throwing a bowling ball onto a very soft mattress. The ball sinks deep initially, but then the mattress slowly rebounds somewhat. The Moon's (and Earth's) crust and mantle also possess a degree of "elasticity" and "fluid-like" properties. After a massive impact, the compressed deep lunar material slowly "rebounds" over millions of years, like in slow motion, lifting the crater floor. Additionally, molten rock (magma) from the impact can surge up through fractures, filling the crater floor like concrete, further leveling and raising it.

2. Abnormally Flat or Even Slightly Convex Crater Floors

What do we see? The floors of some craters are not only shallow but also flat as a parking lot. Stranger still, the central areas of a few craters even bulge slightly upwards, forming a "convex" surface.
Why does this spark imagination? This is even more intriguing. A flat crater floor evokes thoughts of "artificially paved ground." A slightly convex floor resembles the curvature of a massive "dome structure," perhaps formed by internal pressure or its own design. Doesn't that sound exactly like an underground base from science fiction?
The scientific explanation? The flat floor is primarily the result of magma flooding, as mentioned above. During the Moon's more geologically active early period, magma surged up through fractures created by the impact, flooding the entire crater floor. Upon cooling, it formed vast, flat plains of basalt (this is how the large, dark "lunar maria" we see were formed). As for the central bulge, besides the possibility of slight overcompensation during isostatic rebound, it could also be due to subsequent geological activity. For example, as the Moon cooled and contracted overall, it might have squeezed the crater floor crust, causing it to "arch" upwards.

3. The Discovery of "Mascons"

What do we see? This is perhaps the most "hardcore" piece of evidence. In the 1960s and 70s, scientists noticed something strange while tracking lunar orbiters: when probes flew over certain large, circular "lunar maria" (which are actually ancient, massive impact basins filled with lava), their orbits would inexplicably dip slightly, as if tugged by an extra gravitational pull. This indicated that the material beneath these areas was abnormally dense and concentrated in mass. Scientists termed these features Mass Concentrations (Mascons).
Why does this spark imagination? Wow, something incredibly dense underground? That sounds like buried massive metal structures, abandoned alien spacecraft, or high-tech bases! This is almost the core evidence for all "Hollow Moon" or "Moon base" theories.
The scientific explanation? The current mainstream scientific explanation is that when a massive asteroid impacts the Moon, it not only punches through the lighter crust but also "excavates" denser mantle material from deep below. This high-density mantle material fills the deep parts of the crater. Subsequently, it gets covered by erupted basalt lava, which is also relatively dense. Consequently, the total mass of this area becomes significantly greater than the surrounding regular lunar crust, creating a gravitational anomaly. Simply put, it's a natural clump of denser rock, not artificial metal.

4. Peculiar Geometric Shapes

What do we see? The outlines of a few craters, under specific lighting conditions, appear not perfectly circular but show hints of a hexagonal shape.
Why does this spark imagination? Natural impacts produce shockwaves that radiate outwards uniformly, logically forming circles. The appearance of regular geometric shapes easily suggests artificial design.
The scientific explanation? This is actually related to the "jointing" or fracturing of the crust. The Moon's lithosphere isn't a solid slab; it has a natural network of faults and fractures. Over vast geological time, these fractures may have formed grid-like or hexagonal patterns (similar to how dried mud cracks into polygons). When a massive impact occurs, the shockwave energy preferentially travels along these pre-existing, weaker fracture lines. The resulting crater rim can then exhibit a roughly hexagonal outline instead of a perfect circle.

To Summarize

Overall, these intriguing craters do appear somewhat "eccentric" in their appearance and physical properties.

From an intuitive perspective, their flatness, immense size, shallowness, and even gravitational anomalies combine in a way that easily fires the imagination, conjuring images of grand artificial structures from science fiction.
From a scientific perspective, each "anomalous" phenomenon has a plausible explanation grounded in geology and physics, such as lava flooding, gravitational rebound, and crustal fracturing. Although these natural processes are lengthy and complex, they are fully capable of sculpting the strange landscapes we observe today.

So, the next time you come across discussions about lunar bases, you'll understand why people think that way. While these speculations are undeniably cool, all the evidence so far points to the Moon itself – our familiar yet enigmatic neighbor – whose own geological activity is far more masterful and awe-inspiring than we might imagine.