It's a bit tricky, but I will try and explain.
1.Make a huge elevated section
2. On top of it, place three walls of tank
3. Fill the area with water that you want.
4. Lower the water inside until it is about the level of the viewing area height.
5. Flatten the terrain around the edges of the tank wall and it will let the water be seen through the tank walls.
Hopefully this makes sense. If not, feel free to ask a more specific question about a step and I or someone would be happy to help