What causes ecological damage in Alpha Centauri (Alien Crossfire)?

I have been getting a lot of Ecological Damage in my cities for no apparent reason. I have all the base facilities that help Ecology and none of the ones that damage it (like a Genejack Factory). All of my cities are surrounded with forests...in fact, the only terraforming I do (apart from roads) is planting trees.

What's really interesting is that sometimes I get Eco damage mid-turn! Insane amounts!

I'm not sure what causes it, because I checked the Help and I don't have anything that is documented to cause it.

There are 2 things however that I do and I think they might be related:

• Nerve gas pods
• Tectonic missiles

None of them are documented as eco-damaging, but I'm out of options here...

My question is: what are the undocumented sources of eco damage in SMAC?

Edit: The eco damage I get is from cities. I can see the indicator appear in seemingly random cities sometimes mid-turn. This means that I check all my cities at the start of the turn, and I see zero eco damage. Then at the end of the turn there is an insane amount...I summed it up and sometimes my eco damage goes over 100 total.

I can also share a save game file.