Though it's hill-sized not mountain-sized, there is Silbury Hill in England "The largest artificial mound in Europe, mysterious Silbury Hill compares in height

Though it's hill-sized not mountain-sized, there is Silbury Hill in England "The largest artificial mound in Europe, mysterious Silbury Hill compares in height and volume to the roughly contemporary Egyptian pyramids. Probably completed in around 2400 BC, it apparently contains no burial. Though clearly important in itself, its purpose and significance remain unknown." https://www.english-heritage.org.uk/visit/places/silbury-hill/ https://www.english-heritage.org.uk/visit/places/silbury-hil...