Short answer. Ordinarily the owner of the land. The Civil Code treats hidden treasure as belonging to the owner of the property it lies in, and gives half to a stranger who stumbles on it by chance. But it counts as treasure only if no lawful owner appears — otherwise it is simply lost property.
What the law says
By treasure is understood, for legal purposes, any hidden and unknown deposit of money, jewelry, or other precious objects, the lawful ownership of which does not appear.
Civil Code, Article 439 — What Is Treasure. Read the full provision →
Four conditions hide inside one sentence
Article 439 defines treasure as any hidden and unknown deposit of money, jewelry, or other precious objects, the lawful ownership of which does not appear. Each element does work. The deposit must be hidden, so an object merely dropped or left in the open does not qualify. It must be unknown, so a cache the owner of the land knew about is not discovered at all. It must consist of money, jewellery or things of comparable value, which rules out old tools and household junk however interesting. And its ownership must not appear.
The last condition is the one that usually decides it
If a lawful owner can be identified, there is no treasure. Cash in a tin with a passbook, jewellery with an engraving, a strongbox with papers naming a family — all of that is lost property belonging to someone, and the finder's duty is to return it rather than to divide it. People who dig something up tend to reason from the age of the hole, but age is not the test; traceability is. Where an owner or his heirs can be identified, keeping the find is not a windfall, and dividing it with the landowner does not cure the problem.
Who takes it when it really is treasure
The Code gives hidden treasure to the owner of the land, building or other property in which it is found — being the finder on your own land does not entitle you to more, and it does not entitle you to less. Where a stranger makes the discovery by chance, half belongs to him and half to the owner of the property. The words to notice are by chance. A person hired or permitted to search, or one who trespasses in order to dig, is not in the position of somebody who happened upon it while doing something else.
What to do in the first hour
Stop digging and photograph the find where it lies, with the surrounding ground visible, before anything is lifted or cleaned. Note who was present and in what capacity — landowner, worker, contractor, neighbour — because that single fact drives the sharing question. Keep the items together and do not sell or split anything until the ownership question is settled, since a hurried division is very hard to unwind. Objects of cultural or historical significance are governed by special laws rather than by these Civil Code rules, so if what surfaces looks old rather than merely valuable, take advice before it moves.