FRATTOCCHIE, ITALY—After a long archival search and a radar survey, cartographer Tullio Aebischer has uncovered a marble slab in the outskirts of Rome that helped nineteenth-century scientists determine the shape of the earth. Dubbed Benchmark B and left by pioneering astrophysicist Father Angelo Secchi, the slab has a metal plate with a hole in the center. It is identical to a second slab, Benchmark A, discovered in Rome in 1999. By measuring the precise distance between the two benchmarks, which both lie on the ancient Roman Appian Way, Secchi obtained data that resolved a century-long scientific dispute.