Imagine the Empire State Building. Now imagine tipping it on its side, nudging it into the Hudson, and putting out to sea. That was the scale of thing I contemplated one day in late November, as I gaped at the immense navy hull of CMA CGM Christophe Colomb, one of the world’s largest container ships, which stretched above and out of sight on either side of me, on a quayside in Hong Kong. Nearly twelve hundred feet long, it’s bigger than an aircraft carrier and longer than the world’s largest cruise ships. On Christophe Colomb, all of that space goes to boxes. The ship has a capacity of 13,344 TEUs—“twenty-foot equivalent units,” the size of a standard shipping container (although most containers today are forty feet in length). These are stacked seven high above deck and another six to eight below. In cheerful shades of turquoise, maroon, navy, gold, and green, they look like a set of Legos designed for a young giant.

Trying to see where one even boards such a vessel, I noticed a steep aluminum gangway and went up its seventy-four steps, through two hatches, and into the eight-story “castle” that sits above the main deck and houses the ship’s living quarters, offices, and bridge. This was to be my home for nearly four weeks, as I took passage on Christophe Colomb from Hong Kong to Southampton, England, via the Suez Canal.

No passenger liners cover such routes anymore, but many cargo shipping companies still offer a handful of passenger cabins on their freighters, selling travelers what CMA CGM (the French company that owns Christophe Colomb) calls “a thrilling and unforgettable way to discover the great maritime trading routes” for around $130 a day. I had become interested in these sea lanes while writing a book about the world circa 1900 through the life of the novelist Joseph Conrad (1857–1924). Before he became a writer, Conrad spent twenty years as a merchant mariner, sailing chiefly between Asia, Australia, and Europe, and his shipboard experiences informed books such as Heart of Darkness and Lord Jim. When I saw the itinerary of Christophe Colomb today—which plies a regular eleven-week circuit between China and Europe, taking in Hong Kong, the Straits of Malacca, the Suez Canal, the Straits of Gibraltar, and the English Channel—I saw a track that Conrad had often followed and wrote about in his fiction.

In Conrad’s day the world’s great cities had busy commercial waterfronts teeming with longshoremen, manufacturers, wholesalers, carters, innkeepers, ship-chandlers, and prostitutes.…