The foam began washing up on our shores two years ago. At first, it was sparse, resembled barely beaten egg whites, and most of us paid it scant attention. Because it posed no immediate threat, we relegated it to "scientific interest." Over time, however, as it persisted, flowed and thickened into the consistency of properly steamed cappuccino froth, stories started appearing in the news: online, then on television. We traced its origins to deep within the Marianas Trench. But foam is boring, even as it subtly changes hue from ghostly white to green tea. Thus the first images of the foam most of us remember were mechanical, of urban plows pushing it back into the sea. That worked,...