Some kids are exploring by bike, and find a large fenced in area that seems to have been abandoned over 100 years ago.
They look it up on satellite view, and see subtle strange fractal patterns mixed with more chaotic patterns.
They go back out to the site and find a collapsed tunnel under the wall.
Going through the tunnel they find a dead drilling machine (not this big, but looks something like this).
they keep exploring the tunnels and realize that the fractal shapes they saw were the tunneling patterns.
they find an active tunneler with a full load and follow it
it takes them to a central factory where they see piles of various metal stacked up, including gold
before they have time to explore the factory, they set off a silent alarm, and get chased off the site by drillers
They take the gold to a store and have it checked out; it is real gold
the store owner asks questions about where they got it, but they don't tell him
they go back to the site but are secretly followed by the store owner.
They explore the factory and realize that it has been running since the 1800s without anyone tending it!
While they are there they find papers that explain how it happened
People who were reading Darwin and Butler and were acquainted with Babbage attempted to build a "clanking replicator"
but they soon realized it is too difficult
instead they got the help of a rich friend in mining and began building a mining machine / factory
it is an incredibly robust (this fact needs to be very convincing) automated factory that produces mining vehicles
these bring back ore which is turned into more mining vehicles
they want to make the vehicles form evolve, but instead settle on evolving mainly the behavior and proportions of the vehicles
those that bring back the most ore are copied (testing Butler's ideas about the heritability of instinct)
The vehicles (through evolution) began to leave tracks that other vehicles could "read" allowing them to work together as a kind of Tur-Mite (Langton's ant) and develop mining strategies of high computational complexity
Because of a war (World War I?) everyone who knew about the purpose of the project died.
The project was left running without any human supervision
different types have evolved for different cooperative functions
possibly some analogy to aphid farming?
As they learn more about the factory, the store owner gets burglars who sell him stolen jewelry to go to the site and collect the valuable metals
They look at his pictures, realize how much money it is, and kill him to take his share ones he tells them where it is
They go in with trucks to try to get the gold, etc. but run into problems with the drillers and tunnels.
They come back and start blowing stuff up to get to the metals
the kids see them and want to stop them, but the bad guys have big guns
but the kids have figured out a little of what the patterns mean, and dig patterns to tell the drillers where to go to stop the trucks.
they manage to stop the trucks and bring in responsible adults to preserve the factory