"Oho, this is a sculpture!" exclaims Curufin. "Somebody's been at it with carving and buffing tools."

He's wondering the same thing as Conan is. Is there something submerged in the sand under the shell that is causing the quake vibrations? Also, is Nita right? -- are the Voidtreckers supposed to touch the shell for some reason? Is this a trap? Or is this somehow the plan, that some rescuers should come and trigger some event that will save the mer-people? No way to be sure which of these alternatives is the case, or if it is something else altogether.