Welcome to the April A to Z Blogging Challenge! Every year, bloggers from around the world commit to posting every day in April (except Sundays), working through the alphabet one letter at a time. This year, I’m visiting twenty-six fictional alternate Earths — worlds that diverged from our own at some crucial moment and became something wonderfully, unsettlingly different. Think of it like the TV show Sliders, which followed a group of travelers “sliding” between parallel dimensions, never quite knowing what version of Earth they’d land on next. Each day, we visit a new one. Today: U.

The World That Forgot the Sky — and Then Started Arguing About It

There is a theologian in the city of Undermark — third-largest of the Delve Cities, population approximately 340,000, lit entirely by gas lamp and bioluminescent moss cultivated along the great transit tunnels — who has spent the last thirty years of her life writing a systematic defense of the proposition that the Sky is holy. Her name is Maren Solbrecht, and she has been arrested twice, had her manuscripts confiscated once, and been formally denounced by the Ash Council of the Consolidated Delves on four separate occasions. She considers this a reasonably good record, all things considered. The theologians who argue the opposite position — that the Sky is not holy but monstrous, that the great burning was not a test but a sentence, that to look upward with longing is a form of spiritual sickness — have never been arrested at all. They are, after all, the official position.

Welcome to Underground Earth, where the sky is real, the sun still rises every morning, and almost nobody knows.

How the World Went Under

On our Earth, the year 1608 was notable primarily for the invention of the telescope — a device built, with magnificent irony, to look upward. On Underground Earth, 1608 is known as the Year of the Burning, and nobody looks up at all.

The event itself took approximately seventy-two hours. Solar historians on our Earth would recognize it as a coronal mass ejection of almost incomprehensible magnitude — a burst of charged plasma from the sun’s surface so vast that it struck Earth’s magnetosphere like a hammer striking a bell, and the bell, in this case, was the atmosphere. The aurora borealis, ordinarily confined to the polar regions, blazed green and white across every sky on Earth simultaneously on the first night. People gathered in the streets and fields to watch it, which meant that when the radiation intensified on the second day — when the sky turned a color that survivors in the fragmentary written record described only as “wrong” — there were very few people indoors.

The accounts of what followed are sparse, contradictory, and, within three generations, indistinguishable from mythology. What the physical record suggests is this: the CME of 1608 permanently and dramatically elevated the sun’s baseline radiation output, the result of a solar magnetic shift that stabilized at a new and hostile equilibrium. The sky did not return to normal on the third day, or the thirtieth. The elevated ultraviolet and ionizing radiation that had arrived with the storm simply… stayed. Crops failed within a season. Animals that grazed in open fields sickened. The skin of anyone who spent more than a few hours exposed began to show the damage within weeks.

Humanity, being humanity, went looking for somewhere to wait it out. They found the earth itself — the mines and caverns and natural tunnels that had always existed beneath their feet, which now became not curiosities but lifelines. The descent was not orderly. It was not planned. It happened the way most genuine civilizational pivots happen: one desperate community at a time, making the best available decision with the information they had, fully expecting the situation to resolve itself within a year or two.

It has been approximately twelve generations since anyone expected that.

The Delve Cities

The political geography of Underground Earth in its present industrial age is organized around what are called the Delve Cities — eleven major subterranean population centers, each occupying a different geological formation, each with its own government, its own economy, its own dialect of the common tongue, and its own interpretation of the sky.

They are not friendly to one another, exactly, though they trade. The great tunnel networks that connect them — the Delveways, maintained by a rotating consortium of city-state engineers and regarded as the most important shared infrastructure in the world — are as much highways for tension as for commerce. Undermark controls the deepest iron deposits and leverages that ruthlessly in trade negotiations. The twin cities of Ashholm and Caverly have been in a territorial dispute over a contested tunnel junction for sixty years and show no signs of resolving it. The city of Fossura, built into a vast natural cavern system in what was once the Alps, is the closest thing the Delve Cities have to a cultural capital, and it wears that status with the particular smugness of a place that knows it is resented.

What holds them together, loosely and contentiously, is the Consolidated Delves Accord — a treaty framework established roughly 200 years after the Year of the Burning, when it became clear that completely isolated city-states were going to collapse individually and that some minimum of coordination was necessary for collective survival. The Accord has been amended, suspended, violated, reinterpreted, and nearly dissolved more times than its historians can count. It currently requires member cities to maintain open Delveway access, to share certain critical resource data, and to submit disputes above a certain threshold to a joint arbitration council called the Ash Council — named, in an origin story that everyone finds more poignant than they generally admit, for the ash that the first underground survivors carried down with them from the burning surface world.

The Ash Council has no army. Its rulings are frequently ignored. It persists anyway, because even the most combative Delve City understands, at some structural level, that the alternative is worse.

The Church of the Closed Sky and the Solbound Heresy

Ask a citizen of any Delve City what the sky is, and their answer will tell you almost everything about their politics, their neighborhood, their class, and probably how they feel about their parents.

The dominant religious institution across most of the Delve Cities is the Church of the Closed Sky, whose central theological position is precisely what the name suggests: the sky was closed by divine judgment in 1608, the burning was a reckoning visited upon a humanity that had grown too proud, too expansive, too enthralled with the surface world, and the proper response to that reckoning is gratitude for the shelter of the earth and absolute disavowal of any longing to return above. The sky, in this framework, is not a place — it is a lesson. To look upward with reverence is to misunderstand the lesson. To attempt to go above is to repeat the sin that caused the burning in the first place.

The Church of the Closed Sky controls most of the formal educational institutions in seven of the eleven Delve Cities. It has warm relationships with most of the city-state governments, which find its theology of earthward contentment politically convenient. It has not always been kind to its opponents.

Those opponents, loosely organized under the label that the Church gave them — the Solbound, a term originally meant as an insult, meaning those pathetically bound to a dead world above — argue something more complicated and considerably more dangerous. Their position is not simply that the sky is good. Their position is that the sky is still there. That the sun still rises. That the burning was not divine judgment but natural catastrophe, survivable and perhaps even eventually reversible, and that a civilization that has spent twelve generations staring at its own feet has made a spiritual and practical error of historic proportions.

Maren Solbrecht’s thirty years of manuscripts are, essentially, the most sophisticated version of this argument ever written. They are also illegal to distribute in four of the eleven Delve Cities and merely heavily suppressed in the other seven. She continues to write them anyway, by gas lamp, in a room she describes as perfectly comfortable, which everyone who knows her understands to mean that she has not thought about the room in years.

The Surface Runners

Here is what almost no one in the Delve Cities officially acknowledges and almost everyone privately knows: there are people who go above.

They are called Surface Runners, or occasionally Sky Thieves, depending on whether the person using the term finds them romantic or criminal — and in Underground Sun Earth, that distinction is almost entirely class-based. The Ash Council formally banned unsanctioned surface travel in the Accord’s second amendment, 180 years ago, on the reasonable grounds that people who went above tended not to come back in usable condition and the cities couldn’t afford the loss of productive workers. The Church of the Closed Sky supported the ban enthusiastically for its own reasons. The combined effect of legal prohibition and religious condemnation has not, in 180 years, produced anything except a black market.

The suits that make surface travel possible are extraordinary pieces of engineering — heavily layered, reflective, sealed at every joint, with eye protection thick enough to make the wearer essentially blind to detail but alive to light. They are built by a loose network of underground craftspeople who operate in the margins of the industrial economy, sourcing materials through legitimate channels in small enough quantities to avoid notice and assembling them in workshops that move regularly. A full suit costs approximately four years of a skilled laborer’s wages. This means that surface running, in practice, is not available to the desperate or the merely adventurous. It is available to those with resources, connections, or something valuable enough to trade.

What do they find up there? This is the question that the Church of the Closed Sky would most prefer not to be asked, because the answer, passed through the Runner networks in whispered fragments and carefully coded correspondence, is: a great deal. The surface world is not the sterile wasteland of Church teaching. It is damaged and strange and almost entirely unrecognizable to people whose entire frame of reference is stone corridors and gas lamp, but it is not nothing. There are ruins. There are plants — tough, radiation-adapted, nothing like the cultivated underground varieties, but alive. There are animals. And there is the sky itself, which every Runner who has seen it describes differently, and all of them describe the same way: vast, and wrong, and so much bigger than anything they had been told.

The Runners do not, as a rule, stay long. They come back with materials, with information, with the particular expression of someone who has seen something they cannot fully explain to people who weren’t there. Some of them become Solbound advocates. Some of them become very quiet. A few of them go back up again, as soon as they can arrange it, and eventually don’t come back at all, and nobody official records where they went.

The Weight of a Sky You’ve Never Seen

There is a psychological condition recognized in the medical literature of the Delve Cities — informally, because the Church of the Closed Sky has blocked its formal classification in most jurisdictions — called Vault Longing. Its symptoms are not dramatic. A persistent melancholy. An unusual amount of time spent looking at ceilings. A tendency to collect objects described in pre-Burning texts: drawings of trees, fragments of written accounts of weather, the word “horizon” copied out in careful letters and kept somewhere private.

It is extremely common. The estimates of its prevalence range from 20 percent of the population in pro-Church studies to over 60 percent in Solbound-adjacent research, and the range itself tells you most of what you need to know about the state of underground epistemology.

What is consistent across all the estimates is this: the people who experience Vault Longing have never seen the sky. They are not mourning a personal loss. They are mourning something abstract, inherited, written into them by twelve generations of compressed cultural memory and the faint insistence of a world above that continues, despite everything, to exist. The sun rises every morning on Underground Earth. It sets every evening. It has done this, faithfully and lethally and entirely without acknowledgment, for over four centuries. And in the Delve Cities, under the stone, by the gas lamp, people dream about something they have no name for and wake up with the feeling that they have almost remembered something important.

Maren Solbrecht, asked once in a rare interview what she thought the sky actually looked like, paused for a long time before answering.

“I think,” she said, “it looks like the answer to a question we haven’t been allowed to ask.”

She was arrested again the following week. The manuscript she had been working on was not, this time, confiscated. Someone had already copied it.

The copies are still circulating. Down here, that’s how the light travels.

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Join me tomorrow for V — and another world waiting just beyond the edge of what we know.