Requirements: Climb or Fly to get to the top of the mountain, Detect Invisibility to get to Nemo
Basically the island holds a secret, and you need to figure it out... there are a few switches you need to switch to open the final mini area.
If you head to the very top of the mountain, your path may or may not be blocked by lava, if it is you need to work your way around the outside of the mountain area to get past it until you are in 'The Heart of the Caldera' where there is some lava, and kill it, which will switch the direction of the lava. Once it has changed, you can go south, west, north, west, down, east and FLIP SWITCH. Thats the first one and its timed.
Now you need to get all the way to 'By a Waterfall' which is down by the lake, and head west through the hidden exit and FLIP SWITCH.
This should open the door to Nemo's fortress, you can fight various automatons and eventually report back to Harding.
To get the whois, you need to kill the steam engineer (careful, when he gets close to dying or you do, he spills oil on the floor making it so you cannot flee). Killing the engineer short circuits things and opens a door in the far north west of the lair past the librarian. Behind that door is a monstrosity. Kill it, and bring the tiger wing in its corpse to cyrus as proof of the animal experiments.
If you want to fight Nemo himself, you will need a Detect Invisibility, go to Nemo's office and PULL LEVER and then head back to the door
I will pad these notes out next time I do it fresh.