The Deep is a nickname given to the tunnels beneath Riverrock. Descending to unknown depths, the original purpose of this labyrinthine structure seems lost to time. However, Cyrus and the grung now living above seem to believe that something valuable is concealed here. They have dedicated much time and resources--and lost many lives--in pursuit of reaching the bottom. Those who venture into these tunnels are usually called Divers.
Ingress and Egress
The only way to enter the Deep seems to be down a spiral staircase in Riverrock's Low Caste living quarters. At the base of the staircase is a circular room, at the center of which is a ring. This ring--carved out from the ground around it--contains a console with a large sapphire embedded within. Placing one's hand on the sapphire immediately turns the entire platform into a pool of water. Dropping into the water quickly reveals that it is, apparently, only three feet deep and suspended in midair.
The ceiling above--which was water moments before--appears entirely solid from the hallway beneath. All that drop through the water find themselves with a sapphire in hand. Crushing this gems will instantly transport the holder back to the platform from which they entered.
Strangely, the tunnels do not seem to exist when approached from any other direction but this.
The tunnels are host to all manner of traps and guardians. Many expeditions have taken place over the years, and no small number of grung and lizardfolk have died in the Deep. Many of the guardians carry artifacts of strange design and power, the likes of which have proven valuable enough to justify the risks of entry.
On several occasions, Divers have surrendered their sapphires to creatures in the Deep, whether because they were killed, deceived, or stolen from. Dangerous creatures have escaped and gone on to wreak havoc upon Riverrock before being defeated by response teams. The loss of one's sapphire is generally considered a death sentence, as there is no known way to return to Riverrock without one. After the first attempt by a trapped individual to steal the sapphire of another Diver, Cyrus decreed that anyone without a sapphire should be killed on sight.