What's this obelisk in Marin?

Click here for the satellite image.

OK, so today I cycled into the Marin Headlands to find out what this is:

It's been bugging my friend Josh for some time. You see, he can see this thing from his home in Mill Valley. It's a great white obelisk on a 1000' hill top. It stands inside a circle of 'mailboxes' and makes a humming noise. I took lots of photos of it, and I now know what it is. So just to be cruel, I emailed him this photo of the thing without explanation. Now it seems even more mysterious to him.
Tomorrow I'll post the explanation.