Release: 31/03/2016
Factorio-Version: 1.1.X
Original Author: iamseph, https://forums.factorio.com/viewtopic.php?f=93&t=22790
Note: This is an update to 0.15+, all credit goes to iamseph for making this mod.

This mod adds extra tier of worms, Colossal.
The higher tier worms are bigger, tougher and attack with a long-ranged area-effect acid splash
Most importantly: Worms can come for you!

Starting from evolution factor of 75%, Big, Behemoth and Colossal worms gain the ability to dig underground and slowly tunnel towards your base (they are attracted to the highest pollution area).

When they find your structures they will surface beneath them, destroying them from below before surfacing to spit acid splashes at nearby military targets.

This late-game change shakes up your turtling, making even the best-laid defences vulnerable to attack.

Known issues:
- Worms won't target non-military structures by themselves. Turrets are hardcoded to act this way, so this can't be fixed, unfortunately.
- POSSIBLY CAUSES MULTIPLAYER DESYNCS. Under investigation.

Videos:
Base attack video: https://youtu.be/AAVUZG96QHk
Size demonstration video: https://youtu.be/D0BlNfSWSD4
JD Playthough: https://www.youtube.com/watch?v=u4MeRQ-_f8k&list=PLnIE0W-m629dg0TU69YK8XF8Z6EoBHFFs

Counterplay options:
- Research new Orbital Seismic Scanners tech to be aware of worm movements.
- Pave area with player made tiles. It severly hinders their attempts at surfacing. Higher "Walking Speed Modifier" of a tile improves these effects.
- Have Construction Robots in endangered areas. Worms will die if they can't surface within set amount of attempts, so keeping buildings from being destroyed will prevent worms from surfacing.
- Worms will grow and burrow only if within pollution range. Either clear area surrounding your base or invest in Efficiency Modules to reduce pollution cloud.
- Place turrets in areas with high pollution. Chances are, your turrets will kill or damage the worm while it is busy surfacing.
- Research and build Thumpers, to force worms to the surface. Create worm kill pits!
- Research Seismic Blast Towers technology for ultimate worm defense.

Under the hood mechanics:
- Each worm type has different number of attacks while trying to surface. If it can't clear space to dig out within those attempts, it will die. These numbers are 6, 12 and 18 for Big, Behemoth and Collossal Worms respectively. Tiles that speed up player movement will reduce these numbers, up to by a third with Reinforced Concrete.
- Each burrowing check, worm decides between attacking inside current chunk and moving to adjacent chunks. Logic is as follows:
1) Move orthogonally towards adjacent chunk with most pollution.
2) If current chunk has more pollution than all adjacent chunks, attack that location.
- Worms that start digging will spawn a Small Worm in their place.
- Collossal Worms do not spawn naturally. Instead, worms have a chance to grow bigger when there is any pollution in the chunk they occupy and evolution factor is high enough:

50% Small Worm -> Medium Worm
75% Medium Worm -> Big Worm
95% Big Worm -> Behemoth Worm
99% Behemoth Worm -> Colossal Worm