Even as the first freezing drop of water trickled beneath our wetsuits, we knew that this dive would be historic. We checked our scuba equipment one last time, and then slowly descended into the deep blue waters off the ancient Mediterranean port of Akko, in northern Israel. Our goal: to find the small snail used in ancient times to produce tekhelet, the brilliant blue dye prized in the Bible—but lost to the modern world.

For the first few minutes, we saw nothing. Then my dive partner and old school buddy Baruch Sterman, a physicist who lives in Israel, purposefully reached forward to grab what looked like a rock. He flipped the “rock” over—it was in fact an algae-covered shell that had blended imperceptibly into its background. Before the dive, Rabbi Elyahu Tevger, the leader of our expedition, had precisely described the mollusk we were searching for and had shown us photos. Thanks to his training, Sterman and I immediately recognized the “rock” as having the classic profile of the snail species—called in Latin Murex trunculus—that gave the ancient world the color blue.