The Horizons Beyond

From nearly every vantage in Lisbon, you can see water. If you can’t see the ocean, you can smell its brine or hear it in the cries of seagulls overhead. It is a city steep with hills, and these can be exhausting if you have your two small children in tow, as I did when visiting Portugal this spring. However, each climb offers the promise of a view, and the higher you ascend, presumably, the farther you can see. Except that’s not how horizons work. We saw only varying shades of blue, where the dark ocean met the light sky at some blurred and distant point. What lay beyond that shimmer?

The desire to find out fuelled centuries of exploration as Portuguese sailors opened trade routes and colonized existing nations, leaving few parts of the world untouched. On one side is a romantic hist­ory of leaving — as...