Imagine that, with all the might of modern militaries, there are still places on Earth simply too deep to reach. Not with bombs, not with drills—not even with imagination unhindered by physics.

The challenge presented by deeply buried bunkers hints at a much bigger story: Humanity's fascination with going underground. Consider the ancient underground cities of Cappadocia in Turkey, where thousands lived hidden from enemies—their homes carved out of soft rock some ten stories below the surface, complete with ventilation shafts, wineries, and chapels. Centuries later, during the Cold War, governments constructed vast subterranean labyrinths to protect leaders and citizens from nuclear strikes, making the underground as strategic as the skies above.

Yet, for all our advances, the Earth’s crust remains an impenetrable mystery just a few kilometers down. Weapons designed to crack bunkers are sometimes stumped by clever engineering—or the sheer depth of rock. And it isn’t just military tech that falls short: the deepest humans have ever drilled is the Kola Superdeep Borehole in Russia, clocking in at a little over 12 kilometers—far shallower than the Earth’s radius of 6,371 kilometers. In relative terms, we’ve barely scratched the surface.

What might lie beneath us, inaccessible and undisturbed? Could the real future of exploration be not in the stars, but in the infinitely layered worlds waiting for us just below?

This article was inspired by the headline: '沒用「碉堡剋星」攻擊伊斯法罕 美軍坦承：太深了炸不到 - 自由時報'.