 This area includes an array of linear dark surface markings that appear to criss-cross.In the 19th century, Franz von Gruithuisen is noted for claiming that this area contained a lunar city (which he called "Wallwerk"), based on his observations using a small refracting telescope.This inference was greeted with considerable skepticism by astronomers at the time, and, indeed, subsequent observations with more powerful instruments demonstrated that this was merely a natural feature.