Armed with shovels, gloves, and searchlights, the team followed their map to the excavation site. The mood was “very pirate,” said Dr. Weber. Dr. Telewski set about digging a neat, square hole.

But when they carved deeper and wider, there were no bottles to be found. “The birds started chirping,” said Dr. Weber, and the sun threatened to blow up its cover. “Morale was low.”

When Dr. Beal buried the seed bottles for the first time, he planned to dig one up every 5 years and make the experiment last a century. But over time, those responsible lengthened the period between the excavations to 10 years, then to 20. Two were slightly delayed: The 1919s were postponed to the spring of 1920 – which Dr. Telewski suspects this may be related to the 1918 flu; and 2020 has been postponed to this year due to Covid-19-related campus shutdowns.

In order to avoid losing the thread during these decades, a kind of ministry for seed keepers has developed in the state of Michigan, where every generation of botanists passes the torch on to younger colleagues.

Dr. Telewski – professor of plant biology at the university and the seventh person in charge of the experiment – dug up his first seed bottle in 2000 with his predecessor Jan Zeevaart, who died in 2009. Several years ago, due to his own mortality, he gave a copy of the card to David Lowry, an associate professor of plant biology who had expressed an interest in joining.

Just a few months later, Dr. Telewski had a stroke. While he has since recovered, “it just showed me how difficult it is to give these things off and keep them a secret,” said Dr. Lowry. Shortly afterwards, Dr. Telewski Dr. Weber, an assistant professor at the university, and Dr. Brudvig invited to get involved too.

Over the years, purely practical decisions by Dr. Beal developed a patina of mysticism. Dr. Beal dug up each new bottle under cover of darkness so as not to be dramatic but just to shield the other bottled seeds from sunlight, which could cause them to germinate ahead of their time, said Dr. Telewski. (The team uses green lightbulbs in their headlights for the same reason.)