[ 10/April/21 Eric asked “I wonder how much it would cost to buy every single piece of land on Earth.”]
If I take the question literally, it is impossible.
There is no conceivable way to buy every piece of land on earth, as others have noted much of it is not for sale.
If you meant how much is land valued by various agencies, then that is quite a separate question.
I have owned land for over 40 years. In that time I have sold some, but not without acquiring a replacement piece first. There is no way I would sell into a market where I could not buy. I suspect many others are like me. The only way to take my land from me would be some form of violence. So the idea that anyone could buy all the land on earth is a total nonsense. As the amount of free land went down, the price would rise exponentially – long before it was worth the risk of trying to take mine.
This is one of many scenarios where the very notion of money, of measuring value in markets, fails in catastrophic ways that impose/expose existential level risk to all.
In an age of automated systems, the very idea of measuring value in markets fails in ways that impose serious risk. A failure to comprehend that is one of the serious risks present in society today.