President Trump’s border wall appears to be on shaky ground — on Capitol Hill and on the rocky, shifty terrain where it’s slated to be built.

Congress is showing signs that it might not approve funding for the wall, forcing the president to all but abandon plans to fulfill his promise for a barrier to keep immigrants from crossing the border from Mexico. But even if Trump tried to erect a 2,000-mile fence, it would have to straddle waterways prone to flash floods that would hurtle boulders at the wall and fault lines that could jerk at any time and bring down massive sections of stone or steel.

First consider the evolving politics. Trump recently demanded that a spending bill to keep the government running include funding for the border wall. He pledged to follow that up with an executive order on border security. Democrats, including House Minority Leader Nancy Pelosi (D-Calif.), who described the wall as “immoral,” balked.

Facing a fight that threatens to shut down the federal government, the president softened his demand that Congress approve funding for the wall. As The Washington Post’s Philip Rucker, Robert Costa and David Weigel report: “With a Friday deadline looming to pass a new spending bill, the Trump administration projected confidence that a shutdown would be avoided. In the face of fierce Democratic opposition to funding the wall’s construction, White House officials signaled Monday that the president may be open to an agreement that includes money for border security if not specifically for a wall, with an emphasis on technology and border agents rather than a structure.”

That’s a lot different from Trump’s promise in his June 2015 speech announcing his run for president: “I will build a great wall. And nobody builds walls better than me, believe me. … And I will make Mexico pay for that wall. Mark my words.” Those words played well with supporters at campaign stops who gleefully answered Trump’s raucous call and response.

Trump: “We’re going to build a wall. Who’s going to pay for it?” Supporters: “Mexico!”

Geologists probably weren’t in the crowd. Aside from politics, they said, an even greater force was set to undermine the great wall: Earth. “In California/Baja, the border traverses nearly the entire Pacific-North America tectonic plate boundary, so the western end of the wall is riding the Pacific plate several centimeters north per year relative to the border on the North American side of the San Andreas fault system,” Austin Elliott, a researcher at the University of Oxford in England who studies earthquakes around the world, wrote in an email.

Elliott is an earthquake nerd who tweets about them seemingly nonstop. Like this:

And this reply to geophysicist Mika McKinnon:

“Any wall that is built straddling the fault lines in this region will have to be able to cope with this ongoing deformation, including being abruptly severed in earthquakes,” Elliott said.

There have been at least three earthquakes in the past 125 years that rocked the U.S. southern border, most recently in 2010. The earliest was 1892, when immigration wasn’t a big issue. Another followed in 1940. The earthquake about seven years ago would have split a wall in two as tectonic plates slid by each other beneath it.

Other geologists are not so sure about the feasibility of building a wall on the scale that the president imagines, the entire border with Mexico, an estimated $21 billion venture.

McKinnon, the geophysicist, wondered in Smithsonian Magazine if the barrier can withstand the sands of time. She used the example of Italy’s Tower of Pisa in examining what can go wrong when builders forge ahead with a project without checking whether its foundation can hold it. The leaning tower is now a tourist attraction and laughingstock on which the Italian government spends millions of dollars to prevent its fall.

The tower is a reminder that marvels of present-day engineering “don’t necessarily stay upright,” McKinnon said. Others on Twitter noted enormous challenges to building a wall.

But the politics of it all might make the entire question moot.

Democrats simply aren’t interested in seeing a wall, and Republicans have been reduced to saying the wall was always kind of a metaphor for better border security. The word “metaphor” was never chanted at campaign rallies.

On the Senate floor, Minority Leader Charles E. Schumer (D-N.Y.) ripped into the notion of building a wall but gave Trump an opening, suggesting that smart technology and law enforcement, such as the use of drones, could work as a way of protecting the border. The suggestion echoed a congressional debate in 2006, when George W. Bush was urged to consider tech over brick and mortar.

Perhaps to the surprise of Trump supporters and immigration opponents, Sen. Lindsey O. Graham (R-S.C.) all but dismissed the idea of a wall. “There will never be a 2,200-mile wall built, period. I think it’s become symbolic of better border security. It’s a code word for better border security. If you make it about actually building a 2,200-mile wall, that’s a bridge too far — but I’m mixing my metaphors.”

And so, before Earth can consume the “great wall” that the president promised and asked contractors for bids to build, politics appears to be whittling it away.

See what it looks like along the border fence between the U.S. and Mexico

Aerial picture taken with a drone of the urban fencing on the border between the US and Mexico at El Nido del Aguila, outskirts of Tijuana, northwestern Mexico on January 26, 2017. US President Donald Trump on Thursday told Mexico's president to cancel an upcoming visit to Washington if he is unwilling to foot the bill for a border wall. Escalating a cross border war of words, Trump took to Twitter to publicly upbraid Enrique Pena Nieto. "If Mexico is unwilling to pay for the badly needed wall, then it would be better to cancel the upcoming meeting." / AFP PHOTO / MARIO VAZQUEZMARIO VAZQUEZ/AFP/Getty Images (Mario Vazquez/AFP/Getty Images)

More from Energy and Environment:

For more, you can sign up for our weekly newsletter here and follow us on Twitter here.