“It’s unbearable!” the president declared. “Those strangers are a threat to our society. Soon, there shall be more of them in this country than ourselves. We must react!” And he smashed his heavy, gnarled fist on the lectern.

So the wall was born. A week after this seminal statement, an army of masons, joined by tons of bricks and mortar, invaded the northern bank of the river and set out to work on the spot. Day after day, the wall sprang into existence, gaining both in length and height, until it became a continuous ribbon of bricks and stones, five metres high, crowned in ominous whorls of barbed wire, running uninterrupted over thousands of kilometres. 

Then the works ceased and the masons were told to go back to their homes, leaving behind the impervious obstacle, like a welt over Earth's crust.

A month, maybe two, elapsed in stillness, as if the strangers had been definitely deterred.

Then the first explosions began.

At first, nobody understood. But after a few days, it became clear: vehicles loaded with explosives were silently barged through the river, then hurled at full speed against the wall. The smoke had not even cleared away that a flow of immigrants rushed into the breach and through the rubble into their promised land.

And no one could have banked on the police, whose ranks had been severely depleted, to stop them.

In the capital city, the government met, and decided they should take still more extreme measures.

A few days after, the masons and all their materials were back. As they had been ordered, they diligently endeavoured to erect another wall, two hundred metres beyond the first one. And when this second wall, as ominous as the first one, was done, a throng of specialists in fluorescent jackets gushed in and began to bore the bricks and move the ground, installing kilometres of wires, slew of cameras, raws of electronic detectors, and other shiny contrivances no one except them had any idea what purpose they could serve.

At last, the last hitch was connected, the last wire powered. The gangs of workers sighed and left. Earth was put back in place, and chugging machines levelled the ground. When their task was done, quietness unheard of since months fell.

They named it “the death corridor”. Nothing, not even the smallest bug, could creep though it undetected. High sensitivity thermal detectors, advanced motion tracking cameras, sensitive pressure sensors, invisible laser barriers scrutinised the area day and night, ready to set off the alarms at the slightest unexpected waft of air. And in control centres hidden deep underground, trained operators watched, 24/24, their eyes locked on monitors, their hands hovering over the buttons that would have triggered a local armageddon.

Time went by.

First it was just a trickle. Then the trickle morphed into a brook, and the brook itself changed into a stream.

The strangers were back.

”HOW?” the President yelled, his face red with rage. “I want to know HOW!”

It turned out dozens of tunnels, some more than ten kilometres long, had been dug under the Death corridor. Some were large enough for pick-ups to drive safely inside. It had taken years for the strangers to complete their work, shovelful of ground after shovelful of ground. How many had died in the process? No one knew. What mattered was that they had succeeded.

The tunnels were flooded with water. But everyone knew it was a matter of months for others to pop up. So the president took a grave decision. Since this land couldn’t be protected from invaders, it was time for the people to leave. On Mars, they would have room to explore, land to tame. It was time for his folk to breathe the air of wilderness again.

They built rockets. Many rockets. And when they were ready to depart, they cherrypicked each applicant. Those whose background was judged unclear were dismissed.

One bright day the next winter, the first rockets took off, carrying the upper crust of the society: ministers, counsellors, congressmen, magnates, CEOs. They all boarded with bright smiles and swaggering gaits.

When they landed at Mars’s main astroport, they wondered why the area was cordoned off with burly, armed guards.

”What’s the matter?” the president asked to the local official that had come to meet him at the foot of the gangway.

“I’m very sorry,” the official apologised, “but our policy has changed. We don’t let unknown, potentially harmful strangers in anymore.”