Box work is a type of formation found in caves. It consists of sheets of iron oxide sticking out of the walls or ceilings of a cave. It's called boxwork because the sheets are usually found near each other, and parallel, looking like a series of boxes sticking out of the wall. It's one of the more common cave formations encountered while caving, and it is quite cool to find something looking so manmade jutting out of the wall of a cave.

Box work forms when soft rock containing iron deposits is worn away in the process of forming a cave, leaving the harder iron behind. I believe, but cannot confirm, that the box shape is because the softer rock is usually iron pyrite, which forms cubic crystals.