The obvious realm is the deep seas -we know little about that. We only took plate tectonics seriously starting in the ‘60s.

There's also a lot of knowledge we used to have and which we've lost. I believe past humans had a lot of knowledge about e.g. ecosystem dynamics before the agricultural revolution which we eventually stopped passing down to younger generations once our lifestyles became fully dependent on our own artificial ecosystems. Now we are slowly (re)learning things about natural systems that we sadly neglected to consider in our hubris during the industrial era. Sadly, since we may have destroyed too much at this point.

similarly https://en.wikipedia.org/wiki/Eel_life_history