Icelandic Thunder Bread, also known as 'Þrumari', is a traditional Icelandic bread that is cooked underground using geothermal heat. This unique cooking method gives the bread a dense and moist texture, with a slightly sweet and nutty flavor. The bread is made with a combination of rye flour, whole wheat flour, sugar, baking powder, and buttermilk. It is typically served warm with butter or smoked fish, and is a popular dish during special occasions and festivals in Iceland.