The Hadron Web (also referred to as the Hadron Power Lines) was a web of energy created by the Tremas Master to trap Adric in, allowing him to access Adric's Block Transfer Computation ability.
Using Adric's computations, the Master created Castrovalva. The web was lethal to the touch and was destroyed when Shardovan fell into it. (TV: Castrovalva [+]Christopher H. Bidmead, Doctor Who season 19 (BBC1, 1982).)