In 2008, a brain was found still preserved in mud. It was not done deliberately tho, it was the wet clay that preserved it.
The human brain dates between 673 and 482 B.C and most likely belonged to a man between 26 and 45 who was hanged and decapitated.
Still, these theories don't fully explain why the brain "didn't turn into mush," Collins said.Source:
(Read "Beyond the Brain" in National Geographic magazine.) (source)
"It's curious, because normally it is one of those organs that degrade quickly," he said. "There must be something going on internally that we don't understand." (source)