It's Lighthouse Day in the United States, so let's stroll over to Spring Point Ledge Light on the breakwater next to Southern Maine Community College in South Portland, Maine. Lighthouse enthusiasts will recognize this as a caisson-style light station, meaning it's a lighthouse built on top of a water-tight chamber called a caisson. In the US, caisson lighthouses were once known as 'bug lights' because at a distance they appeared short and broad, a bit like a beetle on the surface of the water. After the invention of the internal combustion engine, the shape reminded many people of spark plugs, so they started calling them 'spark plug lighthouses.' Most caisson lighthouses are offshore or on dangerously exposed sites, but this is the only spark plug lighthouse in the country that visitors can walk to.