There are multiple underground areas in Elden Ring. One of these areas is known as the Deeproot Depths, and it is where you can complete Fia’s questline if you are going for it. To get there, you need to take a page out of the Dark Souls handbook. Here is how you find Darkroot Depths in Elden Ring.
After making your way through the world, you will start the Radahn Festival. This is where you defeat Starscourge Radahn and gain a new Great Rune. After the fight, a star will crash to the earth near Fort Haight in Mistwood, opening the way to Nokron, Eternal City. Go down to Nokron and travel to the northern section of it past the Mimic Tear boss. Go to where you find jellyfish and proceed to jump down a cliff to gain access to the aqueduct dungeon.
Make your way through the dungeon to where the boss room is. You will need to defeat the boss before you can continue onward. This boss is the Valiant Gargoyle boss and is actually two bosses that you fight simultaneously. After the fight, make your way over to the waterfall. You will find a coffin near it that prompts you to rest. Rest in the coffin, and you will get a cutscene where you take a magical coffin ride and awaken in Deeproot Depths.