A company owned by Osama Bin Laden's half-brother has proposed building a bridge across Mandab Strait on the Red Sea, to link Djibouti to Yemen.
Tarek Bin Laden Construction is negotiating with the two governments about plans for the 28.5km bridge - one of the longest in the world.
The proposed bridge would carry a six-lane motorway and a railway.
Apart from the $70bn cost, critics have pointed out that the project would be sited in an earthquake zone.
New cities would be built at either end of the bridge.
Tarek Bin Laden has spent time lobbying politicians in Djibouti and Yemen to accept his proposal to build the bridge across the Mandab Strait, from Djibouti to Yemen's Perim Island, which is the narrowest stretch of sea between the two countries.
Djibouti Prime Minister Dileita Mohamed Dileita told the AFP news agency his government was not actively involved.
"The project fell on us from the sky with the proposal by Osama Bin Laden's brother, who has a construction company in Saudi Arabia," Mr Dieita said.
"People are talking about it a lot here - the Yemenis are convinced the project will be carried out with Saudi and Emirates' funds to connect the Arab world to Africa."
The new town in Djibouti would be called the City of Light.
"Numerous American, Yemeni and even French businesses are taking part in the project," the prime minister said.
"But the big advantage will be to take millions of African Muslims to Mecca, by train or by bus".
But the bridge would not only benefit pilgrims - hundreds of African migrants drown each year trying to reach Yemen from Djibouti's neighbour, Somalia.
If the bridge were ever constructed, it would be among the world's longest.
The longest existing cross-sea bridge is the 32-km Donghai Bridge in China.