"I have found in a small number of ruins reference to a geological anomaly or place known as "FalZhardum Din". This is intriguing because the term not only appears in a few tablet fragments, but very specifically on ornate metal frames in the deepest reaches of the Strongholds Alftand, Irkgnthand and Mzinchaleft of Skyrim."
The mechanism can only be activated with an Attunement Sphere that can be obtained during the quest "Discerning the Transmundane". Once activated, the mechanism transforms into a stairwell that leads to the ancient Dwemer ruins of Blackreach. These mechanisms seem to be the only way to access Blackreach.
Dwemer Scholar, Thelwe Ghelein managed to decipher the ancient Dwemer phrase "FalZhardum Din" that he found on several of these ornate metal frames. It translated to "Blackest Kingdom Reaches", which of course alludes to Blackreach.