Arkady Strugatsky & Boris Strugatsky's "Roadside Picnic" (novel, first contact): Aliens are to us what we are to ants!
I think its classic status probably has to do with the central thesis: aliens so advanced they won't even notice us if they ever came to earth.
Story summary.This is a first contact story, but we never actually see aliens. What we see are signs of their Visitation at 6 different locations on earth. "Imagine that you spin a huge globe & you start firing bullets into it. The bullet holes would lie on the surface in a smooth curve... all six Visitation Zones are situated on the surface of our planet as though someone had taken six shots at Earth from a pistol located somewhere along the Earth-Deneb line. Deneb is the alpha star in Cygnus."
Zones where they visited are dead places. And deadly ones. And contain a lot of alien stuff. Why did they visit? Why did they turn these places into ruins? Why did they leave all those artifacts behind? Many explanations are offered, but one is a particularly special one - the roadside picnic theory: "Picture a forest, a country road, a meadow. A car drives off the country road into the meadow, a group of young people get out of the car carrying bottles, baskets of food, transistor radios & cameras. They light fires, pitch tents, turn on the music. In the morning they leave. The animals, the birds, & insects that watched in horror through the long night creep out from their hiding places. And what do they see? Gas & oil spilled on the grass. Old spark plugs & old filters strewn around. Rags, burnt out bulbs, & a monkey wrench left behind. Oil slicks on the pond. And of course the usual mess--apple cores, candy wrappers, charred remains of the campfire, cans, bottles, somebody's handkerchief, somebody's pen knife, torn newspapers, coins, faded flowers picked in another meadow... A roadside picnic, on some road in the cosmos."
So the Visitation Zones contain litter from their "picnic". I could not find a satisfactory answer to why 6 parties would almost simultaneously decide to come for picnic on earth?
Story is mostly an adventure. Government claims to own the stuff left behind by aliens. But "stalkers" illegally smuggle a lot of stuff out.
Fact sheet.First published: "Avrora literary magazine in 1972, issues 7-10."