In the late 1990s, I was stumped trying to come up with a user name on ebay... everything I tried had been taken. Among the people I was representing at the time were geotechnical engineers, so the nomenclature of their profession was fresh on my mind. The layers below the surface of the earth (the substratum) are central to what they study and design for, so I typed it in, and it took. Been using it ever since.