I was walking down the Los Gatos Creek trail (a popular walking trail running through San Jose, Campbell, Los Gatos), when I put my foot on this bench to tie my shoe: Whilst doing so, I noticed the unusual manhole cover just next to it: Well I thought it was a manhole cover, but it isn't. It is actually like a column buried deep into the ground and there are several other unusual ones as well along this trail: Notice the Chalk in crevice on the left of this one. A woman on the trail found this one with the chalk already there and went ahead and connected the dots which completed the Skeleton image on the plaque. Here is another: Here is a picture that demonstrates that it is a buried pillar: They have simple replacement ciphers on them. For example the one with the Skeleton says (I put it in spoiler if you want to do it yourself): Spoiler ZKK Sghr Vnqkc hr ats z Okzx All This World is but a play Ad Sgnt sgd Inxetk Okzxdq Be Thou The Joyful Player I haven't done all the ciphers, but would love it someone else interested in this might want to do one and share it. From the research I've done, people don't seem to know who put them there.