Why are there vines over the entrance to Bthardamz?

I am looking for Atherium shards for the key to the Atherium forge.

I went to Bthardamz to search for them. After you go down the stairs, where you can pull a lever and the spinning blades come out, I pulled the one for the bars to open the next room. But when I get to the next door there are vines covering the door. What do I do?

Bthardamz is only accessible during the Quest The Only Cure, which starts at the Shrine to Peryite. From the first linked article:

Bthardamz is an ancient Dwemer ruin located in The Elder Scrolls V: Skyrim. It is the largest ruin in the game. The entire ruin is inaccessible unless the quest The Only Cure has been started.