Digestion Pool
From Lexicanum
Digestion pools are one of the final stages of 'Tyraniforming' a planet
Larger Tyranid creatures will vomit their prey into these huge pools of acid, but most of the biomass is consumed by the Ripper Swarms which follow the main force. The rippers consume all the biomass they can before returning to a pool and regurgitating it. The pools use powerful acids to turn the entire planet's organic material into liquid, which is then transferred through the many miles high Capillary Towers to the Hive Fleet orbiting above where it is used to create more powerful and dangerous lifeforms using the DNA from the planet.
After the planet is fully drained many Tyranid lifeforms simply return to the pools to be melted down themselves and transported back to the hive ships as they have no way of transportation back to the fleet.
