When you want to check a date or a place, the first thing you do is to go look at a clay jar, right? Well, no. You look at a calendar or map.

But people who study shipwrecks do look at clay jars.

Pottery is an ancient craft. Clay containers have been part of daily life probably since the days of Adam and Eve. Take a handful of mud. Mush it into a shape. Dry it until it is hard. Put something in it—like colored pencils, paper clips, marbles . . . well, you get the idea.

Throughout history, there have been all kinds of pottery for all sorts of purposes. Marine archaeologists are especially interested in amphorae. These tall, two-handled clay vessels were used to store and deliver grain, wine, and oil. Their features provide good clues about the age of a ship and where it traded.

SHAPE—Does it seem strange that a container has a pointed bottom? That wouldn’t stand up very well! But remember, most amphorae were for shipping, and the narrow pointed shapes were sturdy and stacked to make good use of the cargo space.

MARKS—Potters stamped the names of their workshops into wet clay. Merchants scratched or painted information about the amphora owner or its contents onto the container. Some marks are easy to read. But it takes an expert to make sense of others.

CLAY—Scientists can often tell where an amphora was made by studying its mineral content under a microscope. For example, tiny black grains could mean its clay came from the region of Naples, Italy.

CONTENTS—Amazingly, some shipwreck amphorae are still sealed. You probably wouldn’t want to sample the pickled fish they might contain. But what people produced, what they ate, and where they traded are important puzzle pieces for researchers trying to understand history.