What the heck is this?

[microbe]

I am at war with Inca, and got pretty much everybody else at war with it. When I approach one of his cities, I find this weird tile next to the city with what looks like four muddy holes.

It's not caused by nuclear attack. Only I am in the modern age.

What is that??

 

[superslug]

Those are bombardment craters.

 

[Adam7Eight]

I think they look cool.

 

[Goober]

I was reallt confused about them as well, but I generally do not bombard, I usually do not build many offensive sea units, I build offensive land units. I don't bombard, I take cities.