The Umbilical is one of the few artifacts with an obvious and easily understood purpose. It is a transportation system that connects the two planets Quake and Opal. On the Opal end, the Umbilical is permanently tethered to the seabed. On the Quake end, it is electromagnetically connected to the surface, but this end is disconnected and automatically retracted from the surface each time the orbit of the Dobelle system makes its closest approach to Mandel (every 1.43 standard years). This period is called "Summertide."