(SPOILER) Need help with ancient relics

[Kriya]

So you know how there a secret door in antonios office and u can go under the city? Well there a few ancient relics there. and one that i missed so i completed the place than completed the vatican city and moved to the next chapter. I teleported back to the city since i missed some. So i went down through the excavation site and got most of them but couldnt get one. It the one with the long swimming tunnels where u have to dive down to pass through doors. And it somewhere up. I see a prompt to use my whip but i cant since im underwater and is there a way to go back up there? I tried by going to Antonios office to get through the secret door but it didnt let me... it literally the last one

[Kriya]

Find out how to get the underwater one. once you drop down into the excavation site, theres a door to the left and behind you. Literally right next to the truck. Will take you right there.