Solving the 'hole-in-the-ground' problem

I needed to create a massive underground structure in my environment,

but the CryEngine ocean is just a flat plane. 

Before, with the default CryEngine ocean. Note the middle terrain filled with water.

After, with Water volumes. The middle terrain no longer filled with water.

- How did I solve this problem?
- Water Volumes

I removed the Ocean first. Then, I created 2 Water volumes, one to cover each side of the centre structure. After creating and matching volume vertices with the terrain, I was able to keep the central area clear of any water.

 

After this, I can use Environment Holes to create some space in the middle terrain for the station.