亘る[wataru] to cross over
幾年にも亘る[ikunennimowataru] extending over a number of years
亘古[kouko] for ever
連亘[renkou] extending in a row

に亘って[niwatatte] throughout
長きに亘る[nagakiniwataru] long (used with period of time)
に亘り[niwatari] throughout