document: The proposal from Buckinghamshire councils includes a 15-mile (24km) tunnel from the M25 to Wendover.
  Current government plans include some tunnels but leaves about eight miles in exposed or partially-covered cuttings.
  HS2 said it had "considered" further tunnelling but stood by its plans.
  Plans for the HS2 between London and Birmingham and then to Manchester and Leeds have been developed by the government under holding company HS2 Ltd.
  It promises to reduce journey times between Birmingham and London from 81 to 49 minutes, and slash the trip to Manchester by an hour, to 68 minutes.
  The new Chilterns Long Tunnel Route report, produced by Peter Brett Associates, was commissioned by Chiltern District Council, Aylesbury Vale District Council, Buckinghamshire County Council and the Chilterns Conservation Board to look at alternative proposals.
  The group said the planned route through the Chilterns Area of Outstanding Natural Beauty (AONB) would result in an "adverse and irreversible impact" on the landscape.
  HS2 Ltd's proposal includes about 13km (8 miles) in a tunnel from the M25 to Little Missenden and 12km (7.5 miles) on two viaducts, in two green tunnels and either on embankments or in cuttings.
  The new plan includes a 24.2km (15 mile) tunnel from the M25 to Wendover, the first 9km (5.5 miles) of which would follow the exact alignment of the government scheme.
  The report concluded a long tunnel was "technically feasible", would protect the landscape and "be a better route operationally".
  It is the second time the group has proposed a tunnel scheme but it said the latest proposal would have less impact on the countryside.
  The cost is estimated at Â£1.85bn, as opposed to the government's Â£1.45bn plan, but campaigners said this was "a small price to pay" to "preserve a beautiful and unique part of the countryside".
  HS2 spokesman Ben Ruse said the firm was "committed to protecting the environment" and a tunnel through the entire AONB had been "explored and given extensive consideration".
  He added: "However, we have clearly set out how the current proposals provide the best balance between route engineering design requirements, cost and the desire to minimise environmental impacts."
summary: Plans for an HS2 tunnel under the Chilterns to protect the area have been announced by campaigners opposing existing proposals for the high-speed rail link.