The Miraz passage was a trade route between Miraz and Alca.
According to a Starfleet Academy simulation scenario, several starships were destroyed passing through Miraz, the warp engine being overloaded, without any sign of attack or malfunction. The Starfleet speculated a subspace phenomenon which would destabilize the core, or a new weapon.
USS Excalibur, while escorting the Nicholas, traced a subspace signal from a hidden Klingon starbase responsible for the destruction, equipped with an extensive subspace radio array used as a long range weapon.
Seeing a violation of the Organian treaty, the Excalibur reported the finding to Starfleet. (TOS video game: Starfleet Academy Starship Bridge Simulator)