Tyneside Metro
In the early 1830s, while mining for coal at Spital Tongues at Newcastle-upon-Tyne, a huge cavern was discovered. It was mentioned once in the local newspaper, and forgotten. Nobody explored it, and the end was covered up.
As far as the rest of the world knew, this was true. But, one man saw an opportunity there. His name was Frederick Latimer. He was an avid technologist, and reckoned that he could build a new city down there. He bought over the mine and had the Victoria tunnel built to link it with the docks. Then he started bringing in the finest minds he could buy with the profits from his coal business.
He had geologists and surveyors investigate the cave, which turned out to be a vast cave system. Then he had civil engineers cut a road down into it from the surface to what was to be the start of the New Town of Newcastle.
The Town didn't start out as much, simply a big red brick building, the Newcastle Technical Institute, with an attached 'manufactory and steam experiment building'. There, bright young boys of the area were trained in the ways of steam and science. Then they were sent out to work for the railway companies that were appearing at this time.
Frederick Latimer was the driving force in those early days, building all kinds of things using materials quietly shipped up the Victoria tunnel at night, and encouraging new experiments. His partner, known only as Mr. Porter, wanted to recieve something for his money. Latimer was distracted with building more of his New Town, to which he added more families to over the years, so he failed to notice the sketchy mining operations that Porter was running with labourers 'borrowed' from the local prison. Another labour source was those who were meant to be transported to Australia as convicts. They were mostly worked to death, and the bodies put down a pit with the mining spoil.
Meanwhile, the British government became aware of this gas lit gem beneath Newcastle. The ensuing conversation was interesting to say the least. Neither party had anything the other wanted, and they couldn't really fight each other, but Latimer took a mad notion that they could survive underground without each other. Porter had to cut a deal to keep things open, and his own profits increasing.
The deal entailed continuing their subterranean research down a variety of avenues which absolutely wouldn't be permitted above ground, keeping a facility for the 'diplomatic services' open, in exchange for a blind eye and even assistance in obtaining supplies. Supplies that included disposable labour. Latimer hated it, and became even more eccentric, saying that he was going to start a new underground civilisation. He started building new secret tunnels and going off for days at a time in the cave system. Porter and the government agents, who'd moved in very quickly after the deal, saw that he was mostly a harmless old man, who wouldn't upset things too much if he was left alone.
Meanwhile, the world above kept moving. The railways soon connected all of England, the Empire grew, and the underground Town grew with it. A rail connection was developed to the Newcastle and North Shiels railway, so they could bring in all sorts of goods, no longer limited by the small passage to the docks. Porter started taking an interest in the Newcastle area, starting a shell company to carry out engineering works in the area. This meant he could conceal further passages to the surface, for both air and transportation, in the very fabric of the town.
More and more people were pulled down into the depths of the cavern, some voluntarily, others much less so. Amongst them were multiple boatloads of Irish immigrants, who thought they were bound for America, yet found themselves slaves deep beneath the ground, mining and working in the ever expanding engineering works. There were around 8,000 souls down there by 1850, around 6,000 of them in involuntary servitude. Small numbers of them disappeared over time, but they were mere rounding errors compared to those that died in accidents and illness.
Latimer continued in his madness, building a competing town deeper in, which he claimed to have designed more scientifically, such that it no longer needed the surface, fueled by coal seam gas and the properties of special mushrooms he'd imported from caves in the mountains of Tibet. During the main thrust of the building of the 'New New Town ', relations were almost friendly between his people and the New Towners. Perhaps underground life was going to blossom into a great civilisation. Latimer's Town, as it became known, was exhibiting high levels of self sufficiency.
Time wound on, steam powered more and more, a middle class evolved, money was made. The New Town worked on an underground railway line to Durham, succeeded, and worked on to Leeds, discovering a new series of caverns on the way. Latimer grew more eccentric, but his Town prospered nonetheless.
The Admiralty had an armaments factory built, and supplied labour for it in the form of whoever they managed to imprison on the Empires worldwide conquest. The New Town's connection to the military grew, due to them being the only customer who could be counted on not to ask questions about where the materiel came from. Latimer's New Town withdrew gradually from all their connections, until people forgot about them. Latimer himself hadn't been heard of in a couple of years in 1867.
On 27 February 1867, half the cavern collapsed, suddenly, completely burying the New New Town. No rescue effort was made, due to the sheer magnitude of stone that fell. Tremors also shook up most of the tunnels, but, barring a few inconsequential buildings, and the railway to Leeds, no major harm was done, and all was soon rebuilt. Periodic earthquakes had led to a habit of overbuilding significantly.
No more was ever thought of Latimer or his Town. New Town moved on, building more tunnels into a network that eventually reached from coast to coast. It became further tied to the military, while still being secret. World War 1 led to huge expansion of the complex and it's capabilities, World War 2 led to expansion beyond anyone's wildest dreams. All of England was riddled with tunnels and underground bases, although most of the actual underground population lived in the North of England.
The Cold War led to the Prime Minister of the time choosing to outfit it as a national bunker system, stacked with supplies to keep the nation underground for a whole year. When the Cold War eased off, it was all mothballed, with very few surface-dwellers knowing the system had ever been. Offshoring and the reduced size of Britain's modern military killed the industrial facilities of the New Town. 50 years passed, and the caretakers died off. Nobody disturbed anything, and thanks to good design, and concrete, it just sat there. Any humans down there were a rounding error.