Donald Trump wants to build an "impenetrable, physical, tall, powerful, beautiful, southern border wall" between the US and Mexico.
But how tall? How powerful? How beautiful? The Republican candidate's big ideas can be small on detail, and the wall is no exception.
The US-Mexico border is about 1,900 miles (3100 km) long and traverses all sorts of terrain from empty, dusty desert to the lush and rugged surroundings of the Rio Grande.
Some 650 miles of the border is covered already by a confused and non-continuous series of fences, concrete slabs and other structures.
Mr Trump says his wall will cover 1,000 miles and natural obstacles will take care of the rest.
So what will he build?
This is a wall we are talking about, not a fence - on that Mr Trump has been clear ("a wall is better than fencing and it's much more powerful").
That rules out relatively cheap options like tall iron fence posts or wire mesh.
Concrete is the obvious choice. Ali F. Rhuzkan, a New York-based structural engineer, estimated in an article for National Memo that a 1,900-mile wall - seemingly Mr Trump's original plan - would require about 339 million cubic feet (12.5 million cubic yards) of concrete - three times more than the Hoover Dam.
There are other reasons the costs would be likely to escalate: Mr Trump's plans require extending the wall into increasingly remote and mountainous regions, raising the building costs substantially.
Adding even more to the expense, the new 1,000 miles would crisscross private land, which would have to be purchased, perhaps by legal force, or financial settlements made with owners.
A study by the Washington Post estimated the cost of Mr Trump's wall would be closer to $25 billion.
And who pays?
Mr Trump insists, emphatically, that Mexico will pay. Mexico's president, Enrique Pena Nieto, insists they won't.
Former Mexican president Vicente Fox said: "I'm not going to pay for that ... wall," using a somewhat unpresidential word to make his feelings on the matter clear
Asked by US journalists how he could force Mexico to pay, Mr Trump suggested he could ransom the country by blocking undocumented immigrants from sending money home, using a provision of the US Patriot Act designed to stop funding for terrorism.
Pressed on whether he could really use anti-terror legislation to ransom Mexico into paying for a wall, Mr Trump has variously suggested other options including increasing the fees on visa applications, charging more for border crossing cards and enforcing trade tariffs.
Asked whether any of his solutions were realistic, he told the Washington Post: "It's realistic if you know something about the art of negotiating. If you have a bunch of clowns negotiating, it's not realistic."
International relations experts have questioned whether the security gain of building a huge wall along the border outweigh the security risk of alienating your southern neighbour.
What about the environment?
Free movement between the US and Mexico is not just a human issue. What would the construction of a wall mean for animals that live near the border?
The US-Mexico border region is a delicate ecosystem with regular animal and bird migrations moving between the north and south of the American continent.
A number of species need to cross the border to mate with their genetically different cousins, including the endangered North American jaguar and black bears, which would be threatened without being able to mate with Mexican bears.
The wall would also have to take into account natural flooding zones as well as large areas of sand, where the ground effectively moves.
Then there is the detrimental impact to the landscape of a massive construction project - digging, road building, and the appearance of a concrete wall up to 50 feet high, notwithstanding Mr Trump's pledge that it will be "very beautiful".
BBC
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