How does alien hostility work?

Aliens become more hostile if you attack them or if you step within one tile of an alien nest.
Aliens become less hostile over time, particularly with one rank of Harmony,
which causes aliens to return to neutral twice as fast;
they also become less hostile if an alien nest actually exists within the borders of your city, eventually becoming friendly.
(Note that allowing a unit to walk over the nest will still destroy it, causing all friendship gains to disappear.)

Neutral aliens, particularly siege worms, seem to attack infrequently for no discernable reason.
I've read rumors that hostility broadly affects alien relations with EVERY civilization, but I don't know yet how much truth there is to this, much less how this interacts with any particular civ's unique situation.

Being right next to (i.e. 1 hex away from) a nest will definitely result in being attacked (because there are imaginary baby aliens in there that need to be protected.) Being 2 hexes from a nest may result in being attacked. Therefore I stay 3 hexes away at all times.

Also if there are 2 aliens adjacent to your unit, the chances of being attacked increases. If there's only 1 next to you you are safe. IMO, this is because the chances of that solitary alien winning a fight are too small, (unless your unit is a defenceless Worker or you're right next to a nest).

Practically therefore it's good to be near mountains, coasts, and other units so that the aliens have less ability to surround you. I had a pod in the middle of a sea of aliens, so I sent multiple units in to get it. It kind of worked because they were harder to surround.

Since chances of winning seems to be a factor, it MAY be that the aliens are less likely to attack if you are on a hill or a forest as these are more defendable (I think).

If there are 3 aliens next to your explorer it's time to go swimming and take your chances that there's no sea dragons around.

Worms need one hex of space or else they'll attack. They only move one hex a turn (so their vulnerable to ranged attacks).

Sea dragons attack anything but they can only move 2 hexes at a time so if you're 3 hexes away you're safe (unless there's another one that you can't see).

On top of all this if you keep on killing them and wiping out their nests then they have a tantrum and turn red. So primitive and childish.