In that show, the Enterprise discovers a series of Federation outposts along the Neutral Zone that have been destroyed. Later, the episode reveals that the Romulan outposts on the other side of the zone have also been destroyed. However, the episode never resolves who or what caused the destruction. A review of dialog for this and other episodes might flush out the villains. Must have been one hell of an explosion.
Sensors indicate no evidence of conventional attack.
Can you determine what happened. It's as though some great force just scooped it off the face of the planet. Before the Borg arrive, the Enterprise surveys a nearby planet. It looks like it was once inhabited. It is identical to what happened to the outposts along the Neutral Zone.