A two-year restoration project to repair bridges and other structures along the 140-mile (300km) Coast to Coast route has begun.
It attracts thousands of walkers and cyclists each year and most of the route is on traffic-free paths such as old railway lines.
The scheme includes repairs to Beamish Railway Tunnel and the restoration of a timber-framed tunnel at Seaton.
The £450,00 scheme is partly funded by the Heritage Lottery Fund.
Charity Sustrans, which owns 42 miles (68km) of former railway track along the stretch, is behind the restoration.
It said it would be working with ecologists to protect wildlife habitats, such as those used by colonies of pipistrelle bats in some of the bridges.