One of the key election planks that vaulted US President Donald Trump to power was his vow to build an "impenetrable, physical, tall, powerful, beautiful" wall at the country's southern border with Mexico that he promises would curb illegal immigration.

Now that he is in the Oval Office, one of Mr Trump's first actions has been to sign an executive order authorising the construction of the wall.

Here's a look at some of the key facets of the ambitious proposal.

WHY DOES MR TRUMP WANT A WALL?

Mr Trump has railed against the impact of uncontrolled immigration into the US, infamously alleging in one of his speeches on the campaign trail that immigrants from Mexico were rapists and involved in drug trafficking and other crimes.

According to the immigration section of his campaign website, illegal immigrants and other non-citizens in US prisons together accounted for around 25,000 homicide arrests.
It estimates that the US' current immigration policy costs taxpayers US$300 billion (S$426.18 billion) a year.

Using data from the Centre for Immigration Studies, the manifesto indicated that there were at least two million convicted criminal aliens in the US and that between 9/11 and the end of 2014, at least 380 foreign-born individuals were convicted in terror cases in the United States.

Mr Trump's campaign singled out immigrants from Iraq and Afghanistan as having values that were different from Americans, highlighting a Pew Research Centre study that suggested the majority of them supported the practice of "honour killings" against women.

It also noted that 62 per cent of households headed by illegal immigrants used some form of cash or non-cash welfare programmes, like food stamps or housing assistance.

And it pointed out that nearly one million illegal immigrants, including nearly 200,000 with criminal convictions, had been ordered deported but remain at large.

Mr Trump's view is that the majority of the illegal immigrants enter the US via the country's border with Mexico. He believes that building the wall will prevent immigrants from entering the country.

WHAT WILL THE WALL LOOK LIKE ?

Mr Trump said in February last year that the wall will only have to be 1,609km long because of natural barriers like mountains.

This would be about half the length of the US border with Mexico.

Mr Trump has also said that the wall will be 10.6m-12.2m high. It has been estimated that building the wall to this specification would require about 9.6 million cubic metres of concrete, three times more concrete that what was used to build the Hoover Dam, reported the BBC.

He has compared his wall to the border wall that Israel built along the 1949 armistice lines.

The executive order signed by Mr Trump on Wednesday (Jan 25) does not provide details on the exact composition of the wall, reported the New York Times. It defines it as "a contiguous, physical wall or other similarly secure, contiguous and impassable physical barrier."

WHAT WILL THE WALL COST?

The cost estimates floated by Mr Trump for building the wall have varied; he has suggested in different interviews that it would cost US$8 billion, US$10 billion and US$12 billion.

But the Washington Post estimated that the cost of building the wall according to Mr Trump's plan would be closer to US$25 billion.

Experts have estimated that the cost of maintaining the wall would exceed the cost of building it in seven years.

Mr Trump will rely on a 2006 law that authorised several hundred km of fencing along the frontier with Mexico. That Bill led to the construction of about 1,126.5km of various kinds of fencing designed to block both vehicles in pedestrians.

Mr Trump has been clear that he will build a wall and not a fence. "A wall is better than fencing and it's much more powerful," he said.

That would rule out cheaper options like iron fence posts or wire mesh.

WHO WILL PAY FOR THE WALL?

Mr Trump sparked outrage in Mexico and delight on the campaign trail with his declaration that "Mexico will pay for the wall".

In a statement on his website at the time of the campaign, Mr Trump's team said that Mexico will submit a one-time payment for the wall.

The site also listed a few ways to make Mexico pay for the wall, including cancelling visas, adding visa fees and enforcing trade tariffs. Mr Trump said that the US will not contribute to paying for the wall at all.

But with Mexico consistently stating that it will not pay for the border wall, Mr Trump is likely to seek alternate ways to extract the required funds.

In a press conference earlier this month, Mr Trump reiterated his claim that Mexico would pay for the wall, but hinted that that the cost would be reimbursed, reported Politico.

"In order to get the wall started, Mexico will pay for the wall. But it will be reimbursed. OK?" Mr Trump told reporters. "It will happen. Remember this, OK?"

He also rubbished reports that American taxpayers would initially pay for the wall, with Mexico subsequently repaying the US for the costs.

House Republicans have said they plan to fund the barrier, but the wall would face other obstacles, such as environmental and engineering problems and fights with ranchers and others who would resist giving up their land.

In the executive order signed by Mr Trump, he has ordered the government to "allocate all sources of federal funds" for the wall. He told ABC news the US government will pay the construction costs first and seek reimbursement from Mexico.