A field generator
A field reactor, also called a containment field generator, was an ancient device over a hundred thousand years old built by a society that once lived in The Waters. It produced a containment field that kept The Waters from disappearing. The reactor was in danger when the Monean mining was putting more pressure on it, causing it to divert power usually used for the containment field to prevent it from being crushed. (VOY: "Thirty Days")