A giant 22-pound hunk of bog butter made 2,000 years ago was just unearthed in Ireland, and although it might not be as tasty as it was in the last millennium or two, you can still eat it. So what the heck is "bog butter?" According to Atlas Obscura:
"Bog butter" is butter that has been buried in a bog. The Irish Times describes it as a "creamy white dairy product, which smells like a strong cheese." The earliest known examples date back almost 2,000 years, but there are records of people burying butter as recently as the 1800s. This one is estimated to be an early example but will be studied further to date it.
Why would anyone bury butter in a bog? Often, to preserve it: butter made hundreds of years ago, without salt, wouldn't last long, but the cool, low oxygen environment of the bog could extend its life. Bog butter is sometimes found encased in wooden containers or animal hide, to protect it as well.
For more, here's a brief history of bog butter.