uncharted water

plural uncharted waters

1. countableuncountablenauticalAn area of sea that has not been surveyed, or whose details have not been entered on nautical charts.

   Near-synonym: abyss

   Synonyms: abyss

2. countablefigurativelyplural normallyuncountableAny unknown and potentially dangerous situation.

   That is the rock that he is in danger of, for he is going recklessly over uncharted waters.

   1900, Walter Hines Page, Arthur Wilson Page, The World's Work ...: A History of Our Time

   2 more examples

   Spain has effectively been a two-party state since democracy was reinstated in 1978 and the current stalemate is unprecedented. For King Felipe, who has been on the throne for only 18 months, these are also uncharted waters.

   2016 February 2, Stephen Burgen, “Spain in uncharted waters as king asks socialist leader to form government”, in The Guardian

   As a country, we now enter what the seven freshmen called “unchartered waters”, with an unprecedented political and constitutional crisis looming on the horizon.

   2019 September 25, Geoffrey Kabaservice, “Impeach Trump? The United States is now in uncharted waters”, in The Guardian

   Synonyms: uncharted territory, terra incognita