Ollas are a gravity fed underground watering system. Each olla has a reservoir of water that seeps into the soil around plant roots as the soil dries out. Placed close to plants, they provide hassle free watering. You can simply bury unglazed clay pots with stoppers, but you need to fill them periodically. We prefer having a chain of ollas fed from a small water tank, so you only need to worry about topping up the tank.