The present day
The small girl sat bruised, bleeding and crying in the gutter. Her grandfather looked up, forlornly, at the great empty stone building towering above them.
It had emerged undamaged from all that this once great city had endured, as if nuclear war and alien attacks had been nothing more than mild summer rain. Now it was the only building still standing in this part of the City.
The great stone edifice was just exactly as her grandfather remembered it, when it first arrived.
The old mans thoughts turned to the past.
Quite some years earlier
They came unheralded.
One second there was vacant land, in decaying urban areas, and the next, impressive stone edifices sat squarely upon the land. Teleported in place from lord knows where. One such edifice was transported for each of the truly major cities of the Earth.
It was thought that someone or something must have come earlier to survey likely locations for the buildings, unless of course those who arranged for their transportation were both omnipotent and omniscient. Which may well have been the case.
All the buildings arrived at exactly the same moment, give or take a milli-second.
They opened for business on the next calendar day after their arrival in strict alignment with normal business hours and conscious of the Earth time zones they had been located in.
Not that the buildings were ever truly welcome, but a few individuals overcame their fears and availed themselves of the services offered within. Nobody that did so ever complained. However Governments, almost universally, discouraged their citizens from using the buildings.
The great edifices did not interface with the cities they occupied. They were not connected to city power sources or water supplies, or sewage, or stormwater systems. No goods or equipment of any kind ever entered them, only the people who sought out their services.
Other than imposing a news blackout and establishing "no go" areas around the buildings, Governments had been powerless to do anything about them or those who occupied them. For the most part authorities chose to ignore the buildings. Also most people were mistrustful of the buildings and the vast bulk of humanity never went inside those doors.
Just a few Governments authorised military attacks against the buildings in their cities. Though there had never been any evidence of force shield protection the edifices proved indestructible. Not so much as a single one of the large stones used in their construction was even chipped. This was clearly no stone of Earth and theories abounded as to their nature. Even suggestions that they represented a fourth or fifth state of matter.
Other Governments, while avoiding crude and useless direct assaults, had made subtle and not so subtle attempts to obtain chips or flakes of the stone material. Or any other material that they could take away to analyse. Frustratingly their efforts bore no fruit and, sooner rather than later, they gave it away.