The Isle of Deriad map in Captains Mode has the ground texture flashing and a symbol appears patterned throughout the map. My guess: it's to do with the lightning/rain weather effects.
The symbol looks like a circle with lines and lettering such as this:
OS: 7x64
GPU: GTX970
CPU: i5-4690k
RAM: 16GB
Motherboard: Z97-A
Storage Device (HDD/SSD): SSD
The symbol looks like a circle with lines and lettering such as this:
OS: 7x64
GPU: GTX970
CPU: i5-4690k
RAM: 16GB
Motherboard: Z97-A
Storage Device (HDD/SSD): SSD