An underwater railway tunnel is now open between the eastern and western parts of Istanbul. The tunnel is the world's first to connect two continents: traveling under the waters of the Bosphorus strait, it joins the Asian and European halves of Turkey's largest city together. It's also the world's deepest underwater railway tunnel of its type, according to Turkish officials, sitting 190 feet (58 meters) below the surface of the Bosphorus.

The tunnel connects Asia and Europe under the Bosphorus

The BBC reports the project was first thought up by an Ottoman sultan in the 1860s, but received more timely backing from current prime minister Recep Tayyip Erdogan. Work on the project started in 2004, but was delayed by archaeological digs after the remains of a Byzantine fleet was discovered in the area. The railway — named "Marmaray" for the nearby sea of Marmara, and capable of carrying 75,000 people per hour in both directions — was finally inaugurated yesterday, on the 90th anniversary of the Turkish republic's creation.

The tunnel is 8.5 miles long, but the distance under the Bosphorus itself is fairly short: only 0.8 miles. It was completed with help from Japan, who sent engineers to the country, and added $1 billion to the project's $4 billion budget. Previously, the Bosphorus could only be traversed by ferry, or on one of two bridges. The AFP news agency reports that 2 million people — in a city of 16 million — cross those bridges each day, leading to terrible congestion. Istanbul's mayor, Kadir Topbas, said the new tunnel will "soothe" that congestion.

Construction was delayed by an ancient submerged Byzantine fleet

But the project has also come under fire inside Turkey. The country sits on a fault line, and the tunnel doesn't have an electronic earthquake warning system. The Guardian quotes Rıza Behçet — an engineer who worked on the project — as saying he "would not get on the Marmaray metro line, and nobody else should either." Other complaints have been aimed at Erdogan directly. The prime minister was once mayor of Istanbul, and his far-reaching development plans for the city — including a third airport, a third bridge over the Bosphorus, and a second tunnel — have faced protests in the past: most notably in June of this year when police violently dispersed protesters attempting to stop the urbanization of one of Istanbul's few parks.