Construction work has started on the first rail tunnel beneath the Bosphorus Straits, linking the European and Asian sides of the Turkish city of Istanbul.
The Turkish Prime Minister, Recep Tayyip Erdogan, attended a ceremony to mark the start of digging work on the $2.5bn project.
Officials say that the tunnel, nearly 14km long, will be strong enough to resist earthquakes measuring up to nine on the Richter Scale.
It is expected to be completed within four years.
From the newsroom of the BBC World Service