Last August, as a team at the North Pole prepared to plunge more than two miles to the bottom of the Arctic Ocean, some of the dozens of specialists who staged the dive engaged in a time-honored ritual: drawing on foam cups, decorating more than 100 of them.

The cups were then gingerly sent into the deep. During the historic dive, led by Russian scientists, the pressure of the surrounding water crushed the cups to the size of thimbles, also squeezing their whimsies of writing and drawing.

Afterward, the tiny cups became instant mementoes of the polar dive, offering striking proof of the descent into an unfamiliar zone and silent testimony to the crushing power of plain old water.

“The real North Pole,” read one cup’s shrunken writing. “Explore the abyss,” another urged.

Deep explorers have made thousands of such keepsakes over the decades, and more recently, schools have joined the fun as a way to drive home some of the peculiarities of a planet where very deep water covers some 65 percent of the surface.