Using Suikosource as a reference is all well and good, but where is this mentioned in the games? Has there actually been an instance where a Rune was shown vanishing into the earth or bottom of the ocean of its own free will? I doubt very much that the Soul Eater would chuck itself into the ocean or bury itself if Tir died. I'm sure that, like the Rune of Punishment, it would seek a new host to feed its hunger. Also, the Beast Rune is like the Sun Rune in that it kept itself in an inanimate object with the only difference being that the Blights never had it on them that we know of. Then there's also the one who has himself being carried around when not being abandoned in caves. The Sovereign Rune wasn't a sword itself but lodged within one. Others have been shifted about between bearers. True Wind vanished when Luc died, but for all we know it could've made a home in one of Iksay's windmills. The Sindar had elaborate facilities designed to contain the Runes and their power, so I doubt that lodging themselves underground or underwater is something that happens by default for all but two of them. I think it's safer to say that they just do as they please when not forced by weird containment methods.
Sent from my Ono-Sendai Cyberspace 7 - Break the ICE