TIBRO is home to a busily competitive society of humans who are proud of their science and engineering.  The nearly indestructible architecture of their cities seems inhuman, however.  Low-ceilinged sleeping areas, ramped entrances, and sloping tunnels between buildings give every indication that they were built for another species entirely.  It seems highly probable
that the descendants of the city builders still live on the planet.  Called "Slinkers" by the human Tibrohs, the shy creatures show no evidence of tool use or advanced organization but do well match the probable anatomy of the missing architects.  They live a nomadic lifestyle, slowly migrating around the planet to avoid what can only be described as "Ghosts".  Resembling the
"Slinkers" in every respect except for their intangibility, the second mystery of Tibro is even greater than the first.  Brave and/or intoxicated Tibrohs have risked physical contact with the "Ghosts" and suffered no ill effects whatsoever.  The "Slinkers" flee from them as if in fear for their lives, though, and legend states that if a "Slinker" ever touches a "Ghost", it turns into one.  No such transformation has ever been recorded on video.