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Historically, tunneling has primarily been accomplished through two main methods: the use of massive boring machines and the cut-and-cover technique. The former is well-known for creating extensive tunnels deep underground, as exemplified by the Channel Tunnel. The latter, suitable for shallower tunnels, involves excavating a trench to build the tunnel within, with soil being replaced once construction is completed. A prominent example of this method is the Circle and Metropolitan lines of London's Underground.