Apart from a few bridges that still work, the infra­struc­tur­al achieve­ments of the Roman Empire exist, for us, most­ly as ruins. With a lit­tle imag­i­na­tion, those his­toric sites give us a clear enough sense of the empire’s sheer might, but if we want to go deep­er, we should then look into the numer­ous Roman con­struc­tions that haven’t sur­vived at all. In the video below from his chan­nel Told in Stone, ancient-his­to­ry YouTu­ber Gar­rett Ryan gives his per­son­al top sev­en “lost Roman won­ders,” begin­ning with Trajan’s Bridge, whose length of more than a kilo­me­ter across the Danube made it the longest bridge ever built at that time: a project of ambi­tions befit­ting a man that his­to­ry remem­bers as one of the “Five Good Emper­ors.” No such sta­tus for Nero, though he did com­mis­sion the Subi­a­co Dams. Nec­es­sary to cre­ate a series of arti­fi­cial lakes beneath the infa­mous ruler’s vil­la, they were the high­est dams in exis­tence until the Mid­dle Ages. Hadri­an’s more pub­lic-mind­ed white-mar­ble tem­ple at Cyz­i­cus in mod­ern-day Turkey was known as unusu­al­ly splen­did even by …