Texas Border Force officials have reportedly discovered a well-hidden tunnel that could have made the proposed US-Mexican border wall redundant.

It seems an obvious problem. One with a long pedigree of solutions, reports news.com.au.

When it comes to castle sieges, prison escapes or even rabbit-proof fences, the solution to finding a wall you cannot climb is to go under it.

US border control agents felt compelled to show President Donald Trump pictures of the earthworks when he toured the area. Photo / AP
US border control agents felt compelled to show President Donald Trump pictures of the earthworks when he toured the area. Photo / AP

And Mexican smugglers have long since cottoned on to this idea, even though US President Donald Trump's enormous border wall hasn't even been built yet.

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Border Patrol agents have apparently discovered an 18m tunnel in the process of construction under the border near Hidalgo, Texas.

Any border wall development would have gone right over the top of it.

Its entrance was concealed under brush on the 9m embankment of the Rio Grande River. It is almost impossible to see, unless on the river itself.

It was such a disturbingly devious idea, US border control agents felt compelled to show Trump pictures of the earthworks when he toured the area.

The entrance to the recently discovered border tunnel found in Texas. Photo / KRGV/CNN
The entrance to the recently discovered border tunnel found in Texas. Photo / KRGV/CNN

The tunnel entrance has been dug on US National Parks and Wildlife property — a recent victim of the Trump administration's government shutdown. It was a move taken in protest over the lack of funding approved for his border wall idea.

According to KRGV/CNN, the president of the Hidalgo County Water Improvement District 3, Orthal Brand Jr — which has a pumping station close to the tunnel — says his work teams are unable to get their heavy equipment down the steep embankment to fill in the entrance.

"You've seen them on the news in Arizona and New Mexico and stuff like that but I've never seen one in the Valley," Brand said. "That's the first one I've seen that's in our back yard."

A woman walks on the beach next to the border wall topped with razor wire in Tijuana, Mexico. Photo / AP
A woman walks on the beach next to the border wall topped with razor wire in Tijuana, Mexico. Photo / AP

Brand also told the Rio Grande Guardian the tunnel was "some impressive work, it is not easy work building a tunnel like that."

"The cliff they dug through is a soft sand. The diggers push a piece of pipe into the earth and as they do they remove the sand. As they do that they keep pushing the pipe in. Once they get far enough in they erect these 4x4s and plywood on each side and the top."

Explosives are expected to be packed into the tunnel and detonated, causing it to collapse.

A crumbling edifice

The problem with walls is, they rarely work.

History is full of them. The Great Wall of China. Hadrian's Wall. The walls of Troy, Rome, Constantinople, Jerusalem.

Then there was the French Maginot line intended to halt any German advance during World War II. And the Berlin Wall.

All were big.

All were expensive.

All failed to achieve what they were built for.

Demonstrators knock down a 'mock' wall on the Northern Ireland/Republic of Ireland border, near Newry in Northern Ireland on January 26. Photo / AP
Demonstrators knock down a 'mock' wall on the Northern Ireland/Republic of Ireland border, near Newry in Northern Ireland on January 26. Photo / AP

Even Israel's enormous concrete and steel edifices intended to isolate its citizens from Palestinians angry about their land being occupied have suffered the same problem.

Those who can't go over it simply go through it — or under it.

Israel's solution? Build another wall.

Grand ideas

Vassar College history professor Joseph Nevins recently told Rolling Stone, walls tend to only make matters worse: "What the wall does, it allows people in Israel, especially, not to have to think about them.

"At the same time, it builds up pressure on the other side, that when it explodes, it explodes with unprecedented strength.

"In the case of the US-Mexico borderlands, a similar type of dynamic is unfolding."

University of Hawaii professor Reece Jones points out walls have effectively been obsolete now for several centuries.

"The purpose of an old medieval wall was to protect from an attack on a city, but today walls are obsolete militarily.

"Missiles fly right over them, aeroplanes go right over a wall, a tank can smash through even the most firm wall in a short period of time.

"Walls don't serve a military purpose anymore. The walls that we see going up today serve a different purpose. They're often a symbol."