Donald Trump wants to build an “impenetrable, physical, tall, powerful and beautiful southern border wall” between the US and Mexico.
Many of his supporters have been galvanised by his plans, echoing his calls for a wall at his rallies.
It is a “yuuge” idea, but the Republican nominee has been accused of being small on detail. Britain’s Skynews came up with this brief analysis of some of the ins and outs.
:: What’s already in place?
The border is 1,954 miles across Arizona, California and Texas, stretching across empty plains, to dry desert and the rugged bush of the Rio Grande.
There are already some 650 miles covered by a mix of steel fencing, concrete slabs and other structures.
But Donald Trump says a wall is better and he has vowed to cover 1,000 miles with it, leaving the natural landscape to block out the rest.
It depends what he builds it with. Engineers have suggested concrete as the best option, but iron and wire are cheaper.
Mr Trump claims it will cost anything between $10bn (£7.5bn) to $12bn (£9bn).
But engineers have arrived at higher estimates.
In 2009, the Government Accountability Office said the cost to build a mile of the fence initially averaged between $2.8m (£2.1m) and $3.9m (£2.9m).
But the mountains and deserts may be harder to build in.
Some 1,254 miles of the border is in Texas, but the state only has about 100 miles of wall.
And costs could rise – with much of the land privately owned, it may need to be purchased through financial settlements.
:: Who’s going to foot the bill?
Mr Trump maintains that Mexico will.
Mexico’s President Enrique Pena Nieto insists they will not.
The businessman, who prides himself on the art of negotiating, has suggested various ways of forcing their hand – from increasing the fees for visa applications, to trade tariffs and anti-terror legislation.
But with some feeling alienated by his rhetoric, he must decide if the security benefit is greater than the need to keep America’s neighbour on side.
:: What about the environment?
It is a rich ecological area, with endangered animals and regular animal migrations between the north and south of the American continent.
There are natural flooding zones.
And then there is the impact on the landscape and the challenge of getting the right parties to agree to a new feature – a 50 foot wall?
Source: skynews.com