The title refers to the construction of a submerged floating-tube pontoon bridge/tunnel across the Atlantic Ocean in the novel.

In an alternative history, the United States lost the American Revolutionary War and George Washington was executed for treason.

The divergence point between this world and our own occurred far earlier, however, when the Moors won the Battle of Las Navas de Tolosa on the Iberian Peninsula, on July 16, 1212.

Thus it was that Spain was unable to become unified, owing to the survival of an Islamic presence in its territory, and therefore could not finance the expedition of Christopher Columbus in 1492.

An appearance in connection with a suborbital rocket is also made by an expert (who prefers a mechanical Babbage machine for computing to the electronic kind) named Arthur C. Clarke.