Geomagnetic harddrive

In her recent biography of Sir Christopher Wren – whose towers, domes and steeples appear in the image above – Lisa Jardine describes how she discovered that the London Monument, designed in 1677 by Wren and Robert Hooke together, is actually "a unique, hugely ambitious, vastly oversized scientific instrument" that uses "strategically placed vents and vantage points" to function as a multi-purpose observation deck and lab for measuring atmospheric pressure.
While I was living in Berlin a few years ago, it struck me once that the U-Bahn system could pass, in its own way, for a different kind of "hugely ambitious, vastly oversized scientific instrument" – before I realized, of course, that the Tube, the Metro, the NY subway, etc. – the Beijing underground, Prague, Rome and so forth – all of them could pass for such "scientific instruments."
In other words, those buried urban routes, with all their circuits linked and cross-connected into electrically mechanized networks that passed through mineral deposits and solid bedrock – including the various branches of late-night service that maintained more or less perpetual motion, humming and soaring through manmade canyons beneath parks and plazas and apartment blocks, as if to imply that the global geotechnical industry had been taken over by Athanasius Kircher –


I realized that, in all that tumult of foundations and energy, you could, if you wanted to, listen for the subtle, cello-like moan of distant trains, with their echoes and their friction; and it occurred to me, then, that the whole system, the entirety of the Berlin U-Bahn, could pass for a working model of the universe.
A sonic model, at the very least, of the so-called Cosmic Microwave Background Radiation. A vaulted hum, reverbing back and through itself beneath the city.
Or – and this next idea is only slightly less ridiculous, for you cynics out there – it occurred to me that if the U-Bahn system could somehow be hooked up to massive, earth-anchored magnets, and made, therefore, to produce a magnetic field of its own, that you could transform all of Berlin into a geomagnetic harddrive.
As a sail traps the wind, a *planetary harddrive* would use geomagnetism.
Provided constant motion on behalf of the trains, I thought, and given absolutely gigantic magnets of the right polarity and location, Berlin could start producing its own magnetic field – which meant that any city with a subway could be transformed into a harddrive. Harddrive London. Harddrive Beijing.
Harddrive Moscow.
Of course, it's obvious even to me that you'd have to do quite a lot more than just bury some magnets underground in order to transform a city into a harddrive – you'd need a shovel, for instance, and perhaps some strong anti-manic drugs; but my point is that if Christopher Wren could build a tower that simultaneously memorialized the Great Fire of London even as it acted as a scientific device, then perhaps you could turn *urban infrastructure itself* into a kind of working scientific apparatus.
You could turn all of Berlin into a geomagnetic harddrive.