monumentous
monumental; very large or important
... commencing a monumentous catalogue of the Hemiptera of the world ...
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."
Where it comes from
From monument + -ous. First attested in 1889, probably a new formation, comparable with Latin monumentosum, Portuguese monumentoso (“monumental”).
Adapted from the Wiktionary entry monumentous (CC BY-SA 4.0).