The Xen Healing Pool is a pool found in various locations on Xen. From small to large, emitting a distinct sound and fumes, it heals any creature drinking from it. Any wounds that come into contact with the substance will also be healed.
Along the Half-Life series, the player can heal themself by simply standing in the pool. Their health will be fully recharged after a few seconds. The pool has no restriction on the amount of health it can heal.
As it has the same properties, the Healing Shower likely uses a technology based on it.
The Healing Pool is first seen in Half-Life during the Resonance Cascade, when Freeman is briefly sent to Xen; there he sees Bullsquid drinking from it. In that instance the pool has actually no particular properties while having the same color (no sound and no fumes), as it cannot be really explored, since Freeman is drawn away after a few seconds. In Opposing Force however, when the location is visited by Shephard with the use of the Displacer Cannon, the pool has the proper properties, as he has to travel through the area to go back to Black Mesa.
Healing Pools are often provided during boss battles. During the Nihilanth showdown at the end of Half-Life, Freeman is provided with a Healing Pool hidden behind a rock. Several Healing Pools are also featured in several locations during the Gonarch battle. In Opposing Force, Shephard can often heal himself in a Healing Pool in a safe location while battling the Gene Worm. Each time the Healing Pool is out of range of the boss enemy (hidden by something, in an out of reach place, or found when the boss has already gone in the next location, such as with the Gonarch), allowing the player to recover his health regularly in safety.