I was browsing my survival world today, and noticed the lakes looked funny. I jumped in, and discovered the bottom was green (Grass block)

I am attaching three screenshots. One with a grassy lake, and two showing the grass spreading.

What I expected to happen was...:
I expected dirt under water

What actually happened was...:
Not only was the grass underwater, but it also spread to adjacent grass blocks.

Steps to Reproduce:
1. Generate a world.
2. Find a lake with dirt underneath, and adjacent grass blocks.
3. Wait with the chunk loaded.