Get back alien-artifacts to the game and craft them into space science packs.
Unlock the recipe by researching "Space-science-pack".

Probabilities for loot are:

Biter- and spitter-spawner 70%
Small-worm-turret 60%
Medium-worm-turret 70%
Big-worm-turret 80%

Original mod you can find here: https://mods.factorio.com/mods/engin33rguy/alien-space-science

I just updated this mod to 0.16 and added worms to give some artifacts, too.
The values got nerfed a little.

Useful mod for collecting all the loots: https://mods.factorio.com/mods/hanes/LootChestPlus