There are 2 secret worlds inside The Sims 4.
Here's how to get into Sylvan Glade.
1) Go onto the map (by clicking 'Travel')
2) Search for Crick Cabana in the map (it's the end House on the area of houses above the Museum
3) Search for a "Mystical-looking" tree and view it a few times
4) When the "Water" option appears, click on that a few times, then socialize with the tree a few times and then you'll have the option to explore
5) You MUST click on the following when the pop-ups appear, as this leads you to Sylvan Glade. First, it's 'Follow the Sound', then 'Travel Downstream', then 'Enter the Mist', and finally, 'Travel to Glade'
Here's how to get into Forgotten Grotto.
1) Reach Level 10 Handiness Skill
2) Travel to Oasis Springs and go to Desert Bloom Park
3) Search around for a cave with wooden planks bolted to it and open it
4) Click on the following: 'Wide Path', 'Climb Ladder', 'Step Onto Ledge' and then 'Travel to Grotto'
Enjoy the secret (not-so-secret now) locations!!!