Zerg Creep. Creep producing structures and units can be seen:

Description Edit

Creep is a fundamental Zerg mechanic. Zerg structures can only be created on Creep (except for Hatchery and Extractor) .

If the Creep source is removed from a Zerg structure, so that it is no longer on creep, the structure takes 3 Damage/second, except for Uprooted Spine Crawlers and Spore Crawlers (and Extractors). 1

Larvae can survive the destruction of the Hatchery/Lair/Hive they are attached to, but will die when the Creep beneath them disappears.

Creep is generated by:

[1]: technically a Zerg structure is checked every 0.332 seconds and if it's not on Creep it will take 1 point of damage. Edit