(covers information from several alternate timelines)
Transverse bulkheads were a technology develloped by Lieutenant Tom Paris in an alternate timeline in 2374. Bulkheads were utilized by the ancient steamship RMS Titanic, the honeycomb-like set of bulkheads of which would lower into place in case of a major hull breach, theoretically saving the ship even if half of it were filled with water.
In light of continued Krenim attack, Paris set up emergency force fields between every deck and every section, the force fields were designed to protect most of the crew in the event of a cataclysmic breach. The timeline in which these events occurred was later negated when a temporal incursion erased the Krenim weapon ship from history. (VOY: "Year of Hell", "Year of Hell, Part II")