The "Times Arrow" thing I find interesting, you mean in all these hundreds of years no one ever thought to dig underground in that particular spot and they suddenly find Data's head buried there. I find that wholly remarkable.
It would be great if they found his head stuck on a makeshift scarecrow or something. California and Nevada have abandoned mineshafts all over the place, so it's not unusual that they'd still be there, but people would have come across it for sure, like homeless people, explorers, etc.
They found Data's head in a mine abandoned some time before Data was decapitated there in the 1890s.
One possible reason why nobody had found Data's head in 500 years is mentioned in "Times Arrow Part 1":
LAFORGE: Yeah. They found Data's head a mile beneath San Francisco. Been down there about five centuries.
Obviously it is rather strenuous to climb down and then back up a mile. So that is a possible reason why nobody ever found Data's head in 500 years.
But there is a problem. In "Times Arrow Part 2":
GUINAN: I found the Head Surveyor, I found the cavern. You will not be able to get to it. The entrance is on a mine shaft that is the middle of the Presidio on an army base.
I find it a little hard to believe that someone could dig a mine a mile deep on the Presidio army base during the 19th century. It would take a long time to dig a mine a mile deep and the rocks and dirt from the mine would cover a large area. Presumably the mine owners would have to pay the army large rent to make up for the inconvenience. And I don't know if there are any valuable minerals in the San Francisco area.
Possibly the mine was a lot less than about 5,280 feet deep in the 19th century, which would also explain how so many people were able to climb down and back up it in "Times Arrow Part 2". Possibly some time the future the ground level there will be raised by hundreds or thousands of feet. But the Presidio area of San Francisco is seen in a number of
Star Trek movies and tv episodes. And as far as I remember there isn't a big hill in that area in any
Star Trek production.
Possibly miners dug a mine far from the Golden Gate and outside the military reservation during the Gold Rush years, and the US army later extended the Presidio Military reservation to include the entrance to the abandoned mine, thus making it inaccessible for most civilians. I am not an expert in San Francisco history. And then sometime in our future a lot of rock and/or dirt was piled up in that area, raising a significant new hill.
Since the Presidio covers 2.347 square miles it is possible that the southernmost regions of the Presidio are over a mile south of the Golden Gate Bridge. It is possible that in the era of
Star Trek a tall artificial hill in the southern Presidio area is hidden by even taller buildings immediately north of it, and so doesn't show up in images of the Presidio area in
Star Trek.
The information about the mine in "Time's Arrow" is rather dubious, but possibly Data's head was really resting about a mile deep, greatly helping to explain why nobody found it there in 500 years.