A well-marked 60 inch brick bulkhead
Call it a blessing or call it a curse, but I tend to notice a bunch of weird little random things while out in the world. This sign is just one of them. Instead of being a tiny pin and/or relatively small survey marker, this thing stands out and demands to be seen.
My guess is that this is some ancient undercrossing of what was then Alviso-Milpitas Road, and as the area was built up, it was covered. It makes sense that you wouldn't want to lose something like this. The money saved in not having to rip up the road or tunnel deeply underneath it would be worth it.
Considering the proximity to the San Jose/Santa Clara water treatment plant, it's possible this is related. Who knows -- maybe it's even in use. There are some dubious-looking lines on the satellite imagery in this area suggesting it's part of the connection.
Look for the patches of green in an otherwise brown field. Something has to be irrigating and/or fertilizing that stuff.