An easement by necessity, similar to an implied easement, is an easement created by the courts. In fact, an easement by necessity is in many ways a type of implied easement. Courts will only create an easement by necessity when the easement is necessary to the use of some piece of property.
Most often, easements by necessity occur when a certain piece of land is completely landlocked and would otherwise have no other access to a road. For example, if your land were completely landlocked, and the only way to get to your land was by crossing over Bill's property, an easement by necessity would exist providing you access to the road over Bill's property.
However, if at any time in the future other options arise and the necessity no longer exists, neither will the easement. In other words, an easement by necessity can and will only last so long as it is necessary to achieve its result. Given our previous example, if at some later time the city buys some land next to you and puts in a new road that now gives you direct access without having to cross anyone else's property, your previous easement by necessity over Bill's property will cease to exist because it is no longer necessary to achieve its result. In that case, the easement by necessity would be "extinguished".