Brockenbores, or Brockenborings, was a network of tunnels in the northern part of the Shire's Eastfarthing, in the hilly region of Scary. This was one of the more outlying parts of the Shire, which was perhaps the reason that a band of Hobbit rebels led by Fredegar Bolger, hid here for a time when 'Sharkey' took control of the Shire during the War of the Ring.
Description
As depicted in The Lord of the Rings Online