There is a setting within your landscape components ‘Details’ tab which can allow you to turn off static shadow casting. Use the image I have posted as a reference on how to find this setting. Hopefully this gets rid of your shadows of the landscape hole areas.
Landscape Static Shadow
Also, another thing you could try to illuminate your scene on a global scale would be to add in a ‘Skylight.’ This light serves as an ambient fill light for your scene so you can get a better brightness level in really dark areas.
Let me know if these suggestions worked, or if you need further assistance.