Complex sentence: It was reopened during the high mineral market prices of World War I. After World War I, a post-war economic down turn caused the smelter and mine to close again, and by 1930 the population of Humboldt had dwindled to 300.
Split into simpler sentences: It was reopened during the high mineral market prices of World War I. After World War I, a post-war economic down turn caused the smelter and mine to close again. Mines and the smelter closed, and by 1930 the population of Humboldt had dwindled to 300.