Of great importance (3)
(Other definitions for key that I've seen before include "It can open" , "Lock undoer" , "One that locks" , "Lock opener; low island" , "Vital" .)
(Other definitions for earthshaking that I've seen before include "Of great importance; like a quake?" , "Very important or shocking" , "Of huge importance" , "Momentous - hat shrinkage (anag)" , "Of seismic proportions" .)