Short answers, pulled from the story.
A midden is an ancient dump for domestic waste. It may contain animal bones, human excrement, botanical material, mollusc shells, potsherds, stone tools, and other artifacts associated with past human occupation. Archaeologists study middens to reconstruct the diets, habits, and environments of past societies.
Shell middens are rich in calcium carbonate, which makes the deposit alkaline and slows organic decay. This preserves food remnants, organic tools, clothing, and human remains at higher rates than ordinary soil would allow. They also contain detailed records of what food was eaten and processed, alongside fragments of stone tools and household goods.
The Danish naturalist Japetus Steenstrup first used the term køkkenmøddinger to describe shell heaps he studied in Denmark. The related singular word køkkenmødding, meaning kitchen mound, is now used internationally as a scientific term.
Shell middens in the San Francisco Bay Area, built by the Ohlone and Coast Miwok peoples over thousands of years, served as burial sites, ceremonial spaces, cemeteries, and places of prayer. The Emeryville Shellmound between Oakland and Berkeley was estimated at 60 feet high and 350 feet in diameter before its demolition in 1924.
Sambaquis were built in Brazil from around the 6th millennium BCE until the beginning of European colonisation. They are interpreted as waste products of coastal meals consumed by nomadic groups and hunting parties.
Several animal species build structures called middens. The North American red squirrel (Tamiasciurus hudsonicus) typically maintains one large active midden per territory as a food store. The hyrax uses middens as territorial markers, and octopuses pile rocks, shells, and prey bones at their den entrances to conceal them.