If someone finds a very precious treasure on the land. What is the proper thing to do? Does the treasure belong to the finder or to the land's owner?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If this treasure has any signs which suggest that it is buried after the advent of Islam, or it is not known whether it is buried in the pre-Islamic era or after the advent of Islam, then the ruling that applies to it is that of a lost property. If its owner is known, it has to be returned to him, otherwise it should be made known for a period of a whole year and if no one comes to claim it then the person who found it can benefit from it and when its owner comes at any time, then he has to compensate him for it. Imaam An-Nawawi said in his book Al-Minhaaj: 'If this treasure has signs of the pre-Islamic era, then the person who found it has to pay a fifth part of it as Zakat and take the four remaining fifths, on the condition that the treasure reaches the minimum amount liable for Zakat (equivalence of 85 grams of gold or more) and is found in a deserted land, or a property that is cultivated and owned by him. But if a pre-Islamic item is found in a mosque, or in the street then it is considered as a lost property. If it is found on someone's property, then it belongs to the owner if he claims it, otherwise to the one who owned this property before him, and trace its owners until we arrive at the first person who cultivated the land. 

Allaah knows best.