An excerpt from an alien report on the terrestrial landscape, as recounted by Primo Levi.
2.3 ELLIPTICAL CRATERS
The existence of elliptical (more rarely circular or semicircular) craters within certain Cities or in their immediate vicinity was already pointed out in previous Reports. They formed slowly (in the course of five to fifteen years) even in very ancient times near various Cities of the Mediterranean area; but there is no record of their having been observed before the eighth century BC. The majority of these ancient craters were later more or less completely obliterated, perhaps due to erosion or as a consequence of natural catastrophes. During the last sixty years many other craters have formed with great regularity within or close to all the Cities, with an extension superior to 30 or 50 hectares; the largest Cities often have two or more. They never appear on inclines, and their shapes and dimensions are very uniform. Rather than being of a precisely elliptical design, they consist of a rectangle measuring approximately 160 to 200 meters, completed on the two short sides by two semicircumferences. Their orientation appears haphazard, both in respect to the urban reticulate, and to the cardinal points. That these are craters has been clearly recognized on the basis of the profile of their shadows at dusk: their rim is 12 to 20 meters high in relation to the ground, it drops sheerly on the outside, and toward the inside has a declivity of approximately 50 percent. Some of them, during the summer season, emit at times a tenuous luminosity during the early hours of the night.
Their volcanic origin is deemed probable, but their relationship to the urban formations is obscure. Just as mysterious is the weekly rhythm to which the craters seem to be typically subject, and which we shall describe here below.
3. NONASTRONOMICAL PERIODICITY
A certain number of phenomena observed on earth follow a seven-day rhythm. Only the optical instruments at our disposal for a few decades have allowed us to highlight this singularity; therefore, we are not in a position to establish whether its origins are recent or remote, or even if this singularity goes back to the solidifying of the terrestrial crust. It is certainly not an astronomical rhythm: as is well known, neither the terrestrial month (synodic or sidereal) nor the year (solar or sidereal) contains a number of days which is a multiple of 7.
The weekly rhythm is extremely rigid. The phenomena which we shall call OTSD (Of The Seventh Day), and which mainly concern the Cities and their immediate surroundings, take place simultaneously on the entire terrestrial surface; allowances being made, of course, for discrepancies in local times. This fact is not explained, nor have truly satisfactory hypotheses been advanced: just as a matter of curiosity we point out that some observers have formulated the supposition of a biological rhythm. Any possible life (vegetal and/or animal) on Earth, that in this hypothesis would have to be accepted as rigorously monogenetic, would be subject to an extremely general cycle, in which activity and rest (or vice versa) follow each other in periods of six days and one day.
3.1 THE OTSD ACTIVITY OF THE CRATERS
As mentioned, the elliptical craters referred to under section 2.3 are subject to a weekly rhythm.
Every seven days their contours, which normally are whitish, become gray or black within a few hours (generally during the early afternoon hours): they maintain this dark coloration for approximately two hours, and then in about fifteen or twenty minutes they resume the original whitish tint. Only exceptionally has the phenomenon been observed on other days than the seventh. The internal area of the crater does not present appreciable variations in color.
From The Sixth Day and Other Tales by Primo Levi, 1966, translated by Raymond Rosenthal.
Image: satellite view of Rome, including Stadio Olimpico, Stadio dei Marmi and Stadio Flaminio. From Google Maps.