Flour dust sounds harmless, until you learn it is thirty five times more explosive than coal dust and worse than gunpowder. Minneapolis found this out in 1878, when the Washburn A Mill erupted, killing eighteen people, flattening neighbouring mills and flinging debris a mile downriver. The disaster reshaped the industry. Ventilation rules tightened, and wooden paths appeared to keep horseshoe sparks from triggering a repeat.

It was rebuilt, then later abandoned. Then the mill caught fire again in 1991, when campers sheltering in its ruins lit the blaze whose scorched shell now houses the Mill City Museum, and doubles, improbably, as a wedding venue.

VoiceMap's self-guided audio tours trace the milling boom back to this stretch of the Mississippi, explaining how flour ground here built General Mills, Cargill and Land O'Lakes into household names, and why Minneapolis still carries the nickname Mill City.