Aerial image of entrance to The Channel Tunnel (Chunnel) (Eurotunnel), beneath the English Channel, Kent, England, United Kingdom, Europe

Barragem do Alqueva, Rio Guadiana river, Alqueva dam hydroelectric power, Moura, Portugal, Europe

Tunnel entrance to Maseline Harbour, Isle of Sark, Channel Islands, Europe

Shuttle entering and Eurostar leaving the Channel Tunnel, seen from Castle Hill, Folkestone, Kent, England, United Kingdom, Europe

A tourist walking through The Underground German Hospital in Guernsey, Channel Islands, United Kingdom, Europe

Malpas canal tunnel, Canal du Midi, UNESCO World Heritage Site, Beziers region, Herault, Languedoc Roussillon, France, Europe

Eurostar leaving channel tunnel, Sangate, France

Trains leaving and entering the Channel Tunnel, Folkestone, Kent

Shuttle with lorries leaving channel tunnel, Sangate, France

Guernsey Underground German Military Hospital from World War Two, Channel Islands, United Kingdom

Entrance of the tunnel Tunnel de Tarragnoz on the Doubs-Rhine-Rhone-channel at Besancon, Doubs, Region Franche-Comte, France, Europe

Entrance of the tunnel Tunnel at Thoraise in the Doubs-Rhine-Rhone-channel, Doubs, Region Franche-Comte, France, Europe

Man walking through a water channel and tunnel, moss, Galeria de agua, Fuentes Marcos y Cordero, natural preserve, Parque Natural de las Nieves, east side of extinct volcanic crater, Caldera de Taburiente, above San Andres, UNESCO Biosphere Reserve, La Palma, Canary Islands, Spain, Europe

Signs on the trekking trail next to the waterfall in Levada do Caldeirao Verde, Queimadas, Madeira

Titus' Tunnel Cuts Through The Solid Rock Of The Mountain In Two Places To Provide A Water Channel To Prevent Flooding, Cevlik, Turkey

Titus' Tunnel Cuts Through The Solid Rock Of The Mountain In Two Places To Provide A Water Channel To Prevent Flooding, Cevlik, Turkey

Expressway Tunnel Ventilation System Building at dusk, USA, Massachusetts, Boston, Fort Point Channel