Donald Trump’s big, beautiful wall has become the trademark of his presidency. It is the promise that more than any other has energized his base, and riled his opponents, and his dogged attachment to it has now brought a large part of the US government to a historic 25 days of partial shutdown.
The 1954 miles of US-Mexican border is a place of astounding diversity of terrain, of land use, of city and countryside, of ethnicity. It traverses desert, river, mountain and sea.
This is the western most point of the US- Mexico border, on the outskirts of San Ysidro, California, a suburb of San Diego that is home to one of the busiest border crossings in the world. Here, the hopes of thousands of migrants who try to make it to the US every year are often dashed.
Democrats say such a barrier is a waste of taxpayers’ money and a standoff over funding led to the longest ever shutdown of the US government 35 days between December and January.
Mr Trump went on to invoke emergency powers, which would allow him to appropriate military funds and resources, a move likely to be rejected by Congress.
He later clarified that it would only cover half of that with nature, such as mountains and rivers, helping to take care of the rest.
However, construction at five major landmarks along its length has been prohibited and some landowners have gone to court in an attempt to stop building on their property.
Mr Trump’s initial price tag of between $8bn and $12bn (£6.4bn and £9.7bn) for a wall covering half the length of the border was widely disputed.
Mr Trump wants $5.7bn in addition to the $1.7bn already allocated for homeland security, most of which is for new and replacement barriers.
The Department of Homeland Security (DHS) previously estimated a wall spanning half the border would cost up to $25bn, but it has now said it is still looking at options to determine the price tag.
US Customs and Border Protection (CPB) says that, on average, it costs approximately $6.5m per mile to construct a new border wall or replace existing legacy fence.
He once elected, he began talking about a barrier made of steel, so that border agents could see through it.
In October 2017, when the Trump administration revealed eight 30ft tall wall prototypes – they were a combination of concrete and metal.
Since December, Mr Trump has said he does not want to build a concrete wall at all, but instead wants “artistically-designed steel slats”.
And just before the government shutdown, he tweeted an image of the design of his “steel slat barrier”, which he said was “totally effective while at the same time beautiful”.
However, figures show that illegal border crossings have seen an overall decline since 2000. In 2000, 1.6 million people were arrested trying to cross the border illegally, while last year that number was just under 400,000.
Building a concrete structure sea to shining sea is the most expensive and least effective way to do border security.
The “vast increases in families and children coming across our border” was presenting a “unique challenge” to the force.
To apply for refugee status in the US, foreign nationals must obtain permission to enter the country before travelling, but those arriving at the US border are able to claim asylum “defensively” to prevent them from being deported back to a situation of “credible fear”.
Despite Mr Trump’s claims, any new border barrier is unlikely to stop these migrants legally claiming asylum at a port of entry.
Although Mr Trump has blamed the southern border for illegal immigration, most actually arises because people over stay their visas.
Canadians were the highest group of over stayers, according to DHS figures, followed by Mexicans and Brazilians (majority of Canadians and Mexicans enter the US by land, and the DHS Overstay Report only provides air and sea over stay rates).
Mr Trump has claimed 90% of heroin comes across the southern border and that a wall would help the fight against drugs.
Nationwide heroin seizures reached 7,979kg in 2017, with 39% seized at the US-Mexico border, according to the Drug Enforcement Agency (DEA).
Most of the border seizures were in the San Diego corridor – approximately 1,073kg in 2017, a 59% increase on the previous year.
While most of the heroin in the US does come from Mexico, the DEA says the majority of it is smuggled in through legal ports of entry, hidden in privately-owned vehicles or transporter lorries, mixed with other goods.
Only a small percentage of the heroin seizures were between entry points – where barriers exist or are proposed.
In fact, there are already barriers in border patrol sectors with the highest volumes of heroin seizures.