CAMBRIDGE, MASSACHUSETTS — The MIT Technology Review is questioning Donald Trump’s claim that his proposed border wall will cost $12 billion. According to MIT, the cost could be closer to $40 billion.

Trump initially proposed to build a wall along all 2,000 miles of the U.S.-Mexico border, but then revised it down to half of the border, with the rest of the border left obstructed by natural barriers such as mountains.

According to the MIT Technology Review, building a wall 50 feet tall with 10 feet extended underground, at a length of 1,000 miles, will require about 9.7 million cubic meters of concrete, which costs about $9 billion. Since Trump has promised the wall will be reinforced with steel, it will need about 2.3 billion kilograms of steel, which adds $4.6 billion to the cost. The addition of labour costs and other expenses brings the estimated cost to between $27 billion and $40 billion.

Bernstein Research previously calculated in July that a wall of 5 ft wide, 1 ft deep and 1,000 miles long would require a total of 7.1 million m3 of concrete at a cost of over $700 million. It would also require around 2.4 million tonnes of cement at a cost of around $240 million. Labor, land acquisition and construction payments would bring the cost for the wall to between $15 billion and $25 billion.

It is not clear who will be paying for the construction as Trump insists that Mexico will pay for it, while Mexico insists that it will not. Trump has also suggested to reporters that he could ransom Mexico into paying by blocking undocumented immigrants from sending money home, the BBC reported.