Answer: insoluble

AnswerCrossword Clue
INSOLUBLEBeyond knotty
INSOLUBLELike the toughest problems
INSOLUBLELike the problem of squaring the circle
INSOLUBLELike chalk in water
insolublethat cannot be solved / something insoluble
insolubleImpossible to solve
insoluble(of a substance) Incapable of being dissolved