In the Age of Legends, Aes Sedai researchers led by the critically acclaimed scholar Beidomon at the Collam Daan discovered a region of the Pattern with what appeared to be an undivided source of the One Power separate from the True Source. Initial research suggested that this energy could be used by either gender without many of the known limitations of the One Power .
An attempt was made to access this new energy source by boring a hole in the Pattern. The resulting explosion destroyed the Sharom of Collam Daan. What the researchers had found was actually the prison of the Dark One. The resulting hole is known as the Bore, and although it has not allowed the Dark One to escape, it does allow him to influence the world.
Over the next hundred years the Dark One's influence in the world grew and his touch caused a degeneration in what had previously been a peaceful utopian society. With time people turned to the Shadow, openly declaring their loyal allegiance to the Dark One, and rose up against the existing order which eventually culminated in The War of Power.
The Bore was later sealed by Lews Therin Telamon in an attempt to imprison the Dark One. The backlash from the Dark One against this act tainted saidin. The Bore itself is described as a metaphysical point in space where the Pattern is its thinnest. It is due to this thinness that the Dark One is able to exert such control over the reality of the Bore's immediate surroundings.
During Tarmon Gai'don, the Bore was opened completely when Logain broke the remaining seals. Rand al'Thor, linked through Callandor with Nynaeve al'Meara, Moiraine Damodred, and Moridin, sealed the Bore permanently by using a pure form of the One Power to recreate the Dark One's prison anew. A shield made of the True Power protected the One Power from the Dark One's backlash. After this event, the Dark One can no longer touch the world directly
The Nature of the Bore
It is still unclear as to exactly what the nature of the Bore was. It is possible that the Bore was a literal hole in the Dark One's prison not large enough for him to escape completely, causing a concurrent thinness in the Pattern that could be sensed. It is also possible that the Pattern itself is the Dark One's prison, meaning the Bore was a hole in the Pattern and Rand created threads of the Pattern to seal it again.