Known as oud (or oudh), it comes from the wood of the Southeast Asian agar (aquilaria) tree. When the wood becomes infected with a particular type of mold, the tree reacts by producing a dark, scented resin, which is often called "liquid gold". Known as the "Wood of the Gods", oud is the basis of some of the world's most extravagant perfumes. One reason why oud is so expensive is its rarity, by some estimates, fewer than two percent of wild agar trees produce it. Oud is most often a base note, which tends to remain on the skin long after the others dissipate. Since they form the perfume's foundation, base notes are very rich, heavy and long-lasting.