Trekking tours to villages of ethnic minorities are not impacted by infrastructural limitations. There are many landmarks in the province and some of the prominent ones are the following. Muang La is an important Buddhist pilgrimage for Theravada Buddhists in the province. Saymoungkhoune Rattana Stupa located here has a highly revered Buddha image, which is 400 years old and is reported to have supernatural powers. Chom Ong Cave, the longest cave in Laos, is located in Ban Chom Ong, 45 km to the northwest of Oudomxay town and is the biggest known cave in northern Laos. The cave was explored by a team of cave researchers during 2009, 2010 and 2011 and reported to be 18.4 km long and considered as the second longest in Laos and 9th longest in South East Asia. The average dimensions of the cave is 20–25 m in width and 20–30 m in height. It has two passages, one is a stream and the other has fossils and both connect to a large hall which is 100 m long, 30 m wide and of varying height of 30–50 m up to the roof. It has a stream inlet in the north and the stream outflows in the southern end. During the Indochina war, people of the village, who had till then kept the location of the cave a secret, had to use the cave as bomb shelter, and they thought that their prayers to Buddha had helped to protect them from the bombings. Hence, they named the cave as “Phachao Khamtan” or “Khamtan Buddha” Cave where 'Phachao' means "Buddha" and 'Khamtan' means "Valuable Protective Shield". The Baci festival was started even before Buddhism made inroads into Laos, as an animist ritual used to celebrate important events and occasions, like births and marriages and also entering the monkhood, departing, returning, beginning a new year, and welcoming or bidding etc. It is particularly special for ethnic groups of the mountainous region of northern Laos such as Oudomxay Province, though it is now celebrated throughout Laos as a national festival and also in neighbouring Thailand. It a traditional cult festival in which after offering prayers to Buddha, in a formal gathering people tie a white thread (symbolically representing purity) or string on the wrist of their opposites wishing for their well being, ward off ill luck and bring them good luck. The ceremony is held as a part of marriage festival or any auspicious occasion in the family when family members get together.