monumentous

monumental; very large or important

... commencing a monumentous catalogue of the Hemiptera of the world ...

... Finch Parsons, who almost single-handedly tackled the momentous and monumentous task of enlisting patrons; ...

memorable or remarkable; worthy of recording or memorializing.

He kept them to himself, however, and contented himself with remarking to the carpenter that he had never met with a more "monumentous episode."

In 1935, a young LTJG Cecil H. Coggins, while working for the ONI, made a monumentous intelligence discovery.