Riverworld I: Resurrection Day
Spaced approximately every mile along the banks of the great River, these curious structures resemble tables (and are indeed dubbed mesas by the local Spanish-speakers on Resurrection Day) some 50 feet in diameter. They are made of a gray metallic substance flecked with red and stand about five feet tall. The surface is riddled with hundreds of 18-inch depressions; Copias fit perfectly into any of these depressions.
The mesas discharge a blast of blue light three times a day. Any copias placed on the mesa will be found to be filled with food and provisions. Although the copias are unharmed, anyone standing on the mesa during a discharge can expect to be instantly incinerated.