The Miracle Mile is the last-ditch perimeter to try to prevent Kaiju from reaching land. It is closer to shore than the Pan Pacific Defense Corps' cordon distance of ten miles, in which to intercept a Kaiju approaching a city.
As Kaiju swim toward targeted coastal cities from the deep ocean, the Defense Corps deploys its Jaegers to the shallow coastal waters offshore to intercept and kill them before the beasts get to their targets. Experience has shown that the target distance for a successful intercept is ten miles offshore.
Should the Kaiju incapacitate, destroy or evade the Jaeger and cross this limit, it becomes extremely difficult to intercept it before it makes landfall. It was considered a miracle for a Jaeger to stop a Kaiju from coming onto shore should they engage after it breaks the ten-mile cordon distance.