Short answer. You do, as a general rule. Article 503 of the Civil Code classifies subterranean waters found under privately owned land as waters of private ownership. The water beneath your land belongs to you as the landowner, in the same way the surface belongs to you.
What the law says
Subterranean waters found on the same
Civil Code, Article 503 — Waters Of Private Ownership. Read the full provision →
Underground water is part of what you own as landowner
Article 503 of the Civil Code sets out which waters are of private ownership. Among them: subterranean waters found on the same — meaning under privately owned land. The law does not treat underground water as automatically belonging to the State or to the public. When it is found beneath your lot, it is yours, in the same way that the soil and everything attached to it belongs to the landowner. This is consistent with the general Civil Code principle that ownership of land includes the surface and everything below it, within the limits fixed by law.
Other private waters the same article covers
Article 503 groups subterranean water with several other categories of privately owned water: continuous or intermittent waters that rise on private land and run through it; lakes and lagoons formed by nature on private land; and rain water that falls on private land as long as it stays within the boundaries. What these have in common is their connection to private land — they arise on, fall on, or lie beneath land that is already privately held. Waters that leave the land or form on public land fall under a different legal regime.
Special laws may regulate use even if you own the water
Ownership of the underground water under Article 503 does not necessarily mean unrestricted use. The Civil Code is the foundational private-law rule, but water resources in the Philippines are also subject to special regulatory legislation. The State has authority to regulate the extraction, use, and distribution of water, particularly groundwater, through permits and other mechanisms under applicable water laws. In practice, if you plan to drill a well or extract significant quantities of underground water, you may need to comply with regulatory requirements separate from the civil-law question of who owns the water. Ownership under Article 503 and the right to extract without a permit are not the same thing.
What this means if your neighbor is extracting water beneath your land
If a neighbor is drilling into the ground and drawing water from beneath your property — crossing the boundary of your land to reach it — your ownership under Article 503 is relevant. Subterranean water found under your land is yours. Extraction that taps it from beneath your lot without your permission is an encroachment on your property rights. Whether the physical reality of underground water flow makes that a viable legal claim depends on the specific facts — groundwater often does not stay neatly under one parcel. A lawyer can assess the situation and advise whether you have a claim and what steps to take to protect your rights.