We were then passing between Sicily and the coast of Tunis. In the narrow space between Cape Bon and the Straits of Messina the bottom of the sea rose almost suddenly. There was a perfect bank, on which there was not more than nine fathoms of water, whilst on either side the depth was ninety fathoms.

The Nautilus had to manoeuvre very carefully so as not to strike against this submarine barrier.

I showed Conseil, on the map of the Mediterranean, the spot occupied by this reef.

"But if you please, sir," observed Conseil, "it is like a real isthmus joining Europe to Africa."

"Yes, my boy, it forms a perfect bar to the Straits of Lybia, and the soundings of Smith have proved that in former times the continents between Cape Boco and Cape Furina were joined."

"I can well believe it," said Conseil.

"I will add," I continued, "that a similar barrier exists between Gibraltar and Ceuta, which in geological times formed the entire Mediterranean."

"What if some volcanic burst should one day raise these two barriers above the waves?"