From the bustling waterfront of this African port city, Europe appears tantalizingly close: The coast of Spain shows on the horizon just nine miles away. Despite decades of dreaming, no one has been able to bridge the physical divide that opened between the two continents more than 5 million years ago, forming the geological bottleneck to the Mediterranean Sea.

In recent months, however, the governments of Morocco and Spain have taken significant steps to move forward with plans to bore a railroad under the muddy bottom of the Strait of Gibraltar. If built, the project would rank among the world’s most ambitious and complex civil engineering feats, alongside the Panama Canal and the Channel Tunnel between Britain and France.