The utilitarian design and the rivets holding everything together can make it feel like airliners are built to last forever. Despite this illusion of longevity, a commercial airliner can expect to last around 27 years in service. But once they leave an airline’s fleet, there are still millions of dollars worth of components hidden in their fuselage. Now a new report from CNBC has broken down what makes the remains of an airplane so expensive.

When an airline is in the market for a new craft, they’re likely to spend somewhere between $90 million and $300 million for a brand-new plane from Airbus or Boeing. So, they look to their old crafts to recoup some of the costs needed to upgrade and replace aging fleets.

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Once an airline is finished with one of its planes, it might try to find a buyer that has a use for older airliners, like cargo fleets and fire-fighting companies. But, if this isn’t possible, it’s sent to the Arizona desert where it joins hundreds of other aircraft that are waiting to be broken down. As it stands, the boneyards of Arizona currently hold more than 4,400 aircraft in varying stages of disrepair.

Red more: Jalopnik