A foreign traveler road-tripping across Europe might well feel a wave of trepidation before driving a fully loaded modern automobile over a more than 2,000-year-old bridge. But it might also be balanced out by the understanding that such a structure has, by definition, stood the test of time — and, for those with a grasp of the history of engineering, that its ancient designers would have ensured its capacity to bear a load far heavier than any that would have crossed it in reality.

With no scientific means of modeling stresses, as classical-history Youtuber Garrett Ryan explains in the new Told in Stone video above, they just had to build it tough. Key to that toughness were arches, “made of heavy blocks laid over a falsework frame until the keystone was slotted into place.” From the late first century, stonework was supplemented or replaced by brick and Roman concrete, a substance much-featured here on Open Culture.