I've been reading for an Anthropology class about the rise of complex societies and this question came to mind. Poking around on the internet didn't reveal much more than theories about Atlantis and other "lost civilizations." Follow-up question: If the record is sufficiently fleshed out to preclude the existence of undiscovered societies, what was the most recent big discovery in the area of ancient civilizations?
Apologies in advance if this is outside the scope of /r/AskHistorians, or otherwise violates the subreddit guidelines.
Theoretically, it is certainly possible. Improvements in technology and technique can contribute to new discoveries - for example, the use of satellite imaging in archaeology. This can be extremely valuable for fields in which there is a relative paucity of written records, like pre-Colombian America.
Just as likely as discovering previously unknown evidence is the re-interpretation of existing evidence. New work constantly revises older conclusions. An example would be Troy - the site contains ruins from several different eras, from different "incarnations" of the city, and there is still debate about which is the Troy of the Homer's Iliad. This kind of re-evaluation could propose the existence of a previously unacknowledged civilization.
edit - a hyphen
Societies or civilisations? We probably know about most civilisations to some extent, as in most areas it is difficult to lose the remains of a city. There are obvious exceptions such as the Amazon area, or where a current-day city overlays another.
Societies in general - absolutely. For instance we know almost nothing about the society that built Gobekli Tepe in about 9000BC, but it was obviously not the first such site that they had built.