After the Minnesota Iron Company began shipping iron ore from the Vermilion Range through the Two Harbors ore docks in 1884, this port far surpassed Duluth in importance. Increased ore-boat traffic, combined with commercial fishing, made navigation extremely hazardous along the foggy, rocky shoreline of Lake Superior, where many ships went aground or were dashed to bits. Completed in 1892, the Two Harbors Light Station alleviated the danger. It was equipped with a Fourth Order Fresnel Lens made of glass prisms surrounding a light source; the central prism magnified the light, while those above and below refracted it into a single beam. Because kerosene originally powered the light source and was volatile, the keeper's house was designed for added protection: the two-story main house has walls two bricks thick, the adjoining light tower has walls three bricks thick, and the wall between them is five bricks thick in case the kerosene exploded. The fog signal house includes displays on the North Shore fishing industry, and the site also includes the assistant keeper's house and the restored pilot house from the Frontenac, one of many iron ore boats claimed by Lake Superior's treacherous waves.