The Mysterious Island is a second town map, distinct from the first. To unlock it, players must complete the Mysterious Island quest.
The Mysterious Island has more residence lots including Premium lots and Houseboats. There are 4 Element-based monuments that can be leveled up using resources that sims find doing everyday tasks.
Note: Some of these resources can be very difficult to find, but there is no specific technique to finding certain resources besides completing as many tasks as possible. For example planting Bell Peppers, baking cookies, or changing in and out of your swimsuit repeatedly.
The monuments are leveled up after a certain number of the correct resources for that monument are collected. This is a very long term process. Resources are not found quickly, so this is not expected to be done fast. Once the Monuments are all the way leveled up, they will activate the Volcano at the center of the island