Monoliths are mysterious and unexplained alien devices discovered on Planet. A monolith will increase a unit's morale (or life cycle) one level, but an individual unit can receive only one monolith upgrade (hint: consider saving your upgrade for those hard-to-get promotions from Commando to Elite).
A monolith will also completely repair any damaged unit; this can be done any number of times, even if a unit has already received a morale upgrade from a monolith.
Monolith squares can also be tapped for resources, and will produce 2 Nutrients, 2 Minerals, and 2 Energy each turn.