Larry Gilbert has commented on the following diary entries

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Maps, their lineage, and the intrigue of invisible roads over 9 years ago

(LivingWithDragons) "Or maybe there is some big secret about those roads, like an underground government community of aliens...!"

With San Diego government, it wouldn't surprise me. It might actually explain a few things. :-)

Actually, when I mentioned this to my wife (long-time native), she claims there *did* used to be some underground tunnels for pedestrians here, but they were much closer to Balboa Park--and they were all filled in the 1970s to close them off due to public safety problems.

This further reminds me of underground tunnels in my hometown in Oregon, supposedly connecting various parts of the city with a big, central, underground space--all constructed for Civil Defense use (the whole nuclear-war-preparedness thing of the Cold War days). In present times, I think the tunnels have a more mundane use as home to various utility conduits. When I was a schoolkid in 1980 or 1981, I got to take a classroom tour of the tunnels that connected to our school!

And then there is the historic underground neighborhood that was built in Seattle around the beginning of the 20th century. It is still accessible today for the price of a ticket for the Seattle Underground Tour. (I lived in Seattle for 15 years and never once went on that tour; I'm kicking myself.)