As the most known site in Cologne, the cathedral is the biggest of its kind I have ever visited. In addition to its impressive scale, its distinctive black color is what makes it famous. I thought the color might have come from a big fire it survived but it turned out that the industrial revolution was the main culprit. Coal heating and steam engines significantly increased the sulphur content in the air in the mid-19th century. The sandstone, which most of the building is made from, reacted with the sulphuric acid in the air/rain and turns into a dark grey color.
Entrance to the cathedral is free but there are also English guided tours daily at 1030 and 230 for €8.