What type of milk can last for several months without spoiling if it sits unopened and unrefrigerated?

Ultra-high temperature (UHT) milk can last for several months without spoiling if it sits unopened and unrefrigerated. UHT milk is treated with high heat to kill bacteria and other microorganisms that can cause spoilage. 

The process of sterilization allows the milk to have a longer shelf life than regular pasteurized milk, which typically must be refrigerated to remain fresh. Once opened, UHT milk should be treated like regular pasteurized milk and should be refrigerated to extend its shelf life. 

It is important to note that UHT milk may have a slightly different taste and texture than regular pasteurized milk, due to the high heat treatment it undergoes.