monumental

adjective4

1. •
   In the manner of a monument.
2. •
   Taking a great amount of time and effort to complete.

   • Creating a complete map of the human connectome would therefore be a monumental milestone but not the end of the journey to understanding how our brains work. — 2012
3. •
   Large, grand and imposing.
4. •
   Relating to monuments.

   • Monumental construction.