I had read about this, but it's not very likely : "that is to say, a place of a skull: some say Adam's skull was found here, and from thence the place had its name; this is an ancient tradition, but without foundation {m}: it seems to be so called, because it was the place where malefactors were executed, and afterwards buried; whose bones and skulls in process of time might be dug up, and some of them might lie scattered about in this place: for, one that was executed as a malefactor {n}," John Gill Exposition of the Entire Bible, commentary about Matthew 27:33 And also, some stone formations in this place, depending on how the time the sun beat down, and projected the shadow, it looked like a skull.