S: (n) Rubicon (the boundary in ancient times between Italy and Gaul; Caesar's crossing it with his army in 49 BC was an act of war)
S: (n) Mohorovicic discontinuity, Moho (the boundary between the Earth's crust and the underlying mantle) "the Mohorovicic discontinuity averages 5 miles down under oceans and 20 miles down under continents"
S: (n) limit, bound, boundary (the greatest possible degree of something) "what he did was beyond the bounds of acceptable behavior"; "to the limit of his ability"