Once a fountain has been activated, vaccing up some water from it will grant the ancient water effect. The water tank HUD will have a rainbow effect applied to it, and the water tank's stack number will be replaced with a timer counting down from 29 seconds. In this time the vacpack's water supply is unlimited, and the water blobs have a subtle rainbow sheen which leave sparkly particles when it comes into contact with a surface or a slime. Any Tarr hit with Ancient Water is obliterated regardless of the time of day. When the effect wears off, it will leave the water tank with normal water at maximum capacity.
- Fountains offer the same benefits as other water sources; establishing a Teleporter near a fountain could offer convenient access to water (and Ancient Water).
- Splashes from ancient water can kill a Tarr in one hit, regardless of the time of day.
- Before Ponds were changed to provide unlimited water like other water nodes, Ancient Water could not be used to fill them, though the regular water left over from the effect could.
- Tarr do not explode when hit with Ancient Water.