You can progress the conversation with The Arbiter. He also chides you for earlier game choices. Above the crystals, inside of a golden shield is …. There are some strange, lavender-and-silver-glowing crystals growing from the floor. Inside is a small chamber, with a spiral ramp around the room.
And there is a door at the end of a short hallway.
You are now inside the Dome that you saw at the top of the path. After what feels like an eternity walking this path, you finally come to the bottom of the cavern floor… and a door leading into the rocks. Spells are effective but enchanted melee weapons might be the best option. Arrows are not a good idea: too many chances to miss. They are a good source of random Dwemer metal and scrap, if you hae the room to carry them. Perhaps they are the source of the radiation? You will encounter the remains of Dwarven Spiders on the path. This is a long path, now circling gradually to the left and still sloping downwards, just not quite as steeply. The path goes to your right - head that way. On the other side of the short wall in front of you, there is a larger dome in the floor of the cavern, surrounded by loose boulders and outgrowths of stone. Leave the Tunnels as you are approaching the entrance, there is an explosion and the ceiling behind you collapses! Go back to the Lakehouse - avoid any close Centurions! Go back to the Tunnels and go down the path, back to the Smelter area. Note: it can be in any set of underground tunnels.
I went to Rykas, tricked him into following me, and went back to the Lake entrance to the Tunnels. I was hoping that the Jarl would want me to dispose of Rykas - somehow - and I was right. That warning at the top of the above picture? Accusations and Getting the Immaculate Armor. By his body is an Immaculate Dwarven Helmet! There is a Smelter on the right, a low wall in front, and the path to the right. Inside, there is a long, downward-sloping path it makes a right hand turn and continues to slope down. I entered the Tunnels not from my quarters or one of the other shops see Part IV but from the boarded up door near the Lake.