They were the tremors, created by a group of Irishmen, which were felt across the continent.
It was the night that Irish footballers took on the best in Europe. And won.
When records were broken and one of the greatest players of all time, from one of the biggest clubs in the world, was beaten and then ousted from European competition by a product of a Christian Brothers school on the northside of Dublin.
On one March evening in 1984, seven players from the Republic of Ireland played in the quarter-finals of European competitions.
Ronnie Whelan scored twice against Benfica on Liverpool’s road to European glory which ended in Rome. Photo: Steve Hale/The Liverpool Collection
All seven progressed to the semi-finals, three of them (Ronnie Whelan, Frank Stapleton and Mickey Walsh) scored on that astonishing night of March 21, 1984. Six Irish footballers played in the final of a European cup in '84, five of them getting winner's medals.
"We were up against some of the best players in the world, we were not only competing with them, we were beating them, and we were there on merit," says Mickey Walsh, the Ireland international who found success and a level of fame as a striker with FC Porto that lives to this day in Portugal.
His international team-mate Frank Stapleton watched the documentary on Channel 4 about Diego Maradona last week with something of a smile: Stapleton scored the deciding goal in a 3-2 aggregate win for Manchester United at home to Maradona's Barcelona in '84, one of the most memorable European nights at Old Trafford.
"My grandkids are a bit too young to notice, or care, that I played against Maradona, but the Barcelona match, and the Maradona factor, is the one thing that people ask my two sons about," Stapleton says.
"It was by no means the best goal of my career. But it was probably the most important."
The goals of that memorable night at Old Trafford can be seen here:
It's hard (impossible, even) to imagine that landscape now, Irishmen knocking in goals to knock out the likes of Barca and Benfica, as UEFA competitions are, by and large, devoid of an Irish presence.
Just one player from the Republic of Ireland has played in the group stages of the Champions League in the last five seasons (Eoghan O'Connell, one appearance for Celtic in 2016).
An Irishman has not scored in the knockout phase of the Champions League in nine years (Darron Gibson).
Violence and death in Heysel Stadium in 1985 would see English football's presence in Europe ended for six years, but in the previous term, 1983/84, England's clubs were on a high, backboned by some sublime Irish talent.
That season, Liverpool (Ronnie Whelan, Mark Lawrenson and Michael Robinson) and Tottenham (Tony Galvin and Chris Hughton) would win the European Cup and the UEFA Cup respectively, while Walsh's Porto were only denied the Cup Winners' Cup by a one-goal margin in the final.
Chris Hughton and Tony Galvin were part of Tottenham’s 1984-winning UEFA Cup team. Photo: Getty
Long before UEFA dreamed up the group-stage idea, European cups were played on a knockout basis and by March 1984, Liverpool (European Cup), Spurs (UEFA Cup), Manchester United and Porto (both Cup Winners' Cup) had all got through to the quarter-finals.
In late March, it came down to the second leg of the last-eight clashes, all on the same evening.
United were at home to Barcelona, 2-0 down from the first leg.
Liverpool were away to Benfica with a 1-0 lead from the home leg.
Porto were away to Shakhtar Donetsk, but the concession of away goals in a 3-2 first-leg win had them nervous. And Spurs had a 2-0 lead to defend away to Austria Vienna.
An Ian Rush goal had given Liverpool a 1-0 win from the home leg with Benfica, but they headed to Lisbon knowing that their opponents had never lost a home tie in Europe.
Benfica had a home record but Liverpool had Ronnie Whelan, who scored twice in a 4-1 win, with Lawrenson also in the starting XI.
"The place was rocking with 70,000 fans all booing and whistling and barracking, and we stuffed them 4-1," Whelan later recalled.
Across the continent the same day, Vienna was the stage for Tottenham, with Hughton and Galvin starting as Spurs drew 2-2 with Austria Vienna, winning the tie 4-2.
Mickey Walsh and his Porto squad had gone way behind the Iron Curtain for their tie on that day. They had struggled in the home leg with Shakhtar Donetsk.
"We had an Irish ref, John Carpenter, for the home game," Walsh recalls.
"We were 2-0 down at half-time but came back to win 3-2, so the away leg was about trying to stay in it. But Shakhtar scored, so we're at 3-3 and they have the away goals," says Walsh, pleased to rediscover the goal, with grainy footage, on YouTube recently.
Mickey Walsh – who played his club football with Porto – celebrates after scoring for Ireland against USSR in September 1984. Photo: Getty
"It was tough, a crowd of around 50,000, Ukraine in March. Snow, really cold, we all had the bobble hats and gloves. I came on to replace one of our strikers with about 20 minutes to go. We had a free-kick that came off the bar, I was there for the rebound and headed it in and that got us through to the semis.
"That was one of the most important club goals I scored in my career. That one and the goal I scored for Ireland against the USSR a few months later are the two that stand out."
Meanwhile, in Manchester, Maradona's Barcelona came to town with a 2-0 first-leg lead and, they imagined, a clear path to the last four.
Despite the deficit, Stapleton, one of two Dubliners in the team alongside Kevin Moran, was hopeful.
"We played OK in the Nou Camp, one of their goals against us was an own goal and the second was a top-corner effort in the last minute of the game. So we knew we had a chance in the second leg," says Stapleton.
"For the home leg, I had never seen so many people inside Old Trafford. I speak to people about it now and they all claim to have been there that night.
"The capacity was supposed to be 58,000 but I reckon another 10,000 managed to get in, somehow. The atmosphere was electric, the attraction of having Maradona and (Bernd) Schuster playing was special. It was set up for a special night.
"I played with, and against, some great players, but Maradona is in a special category," recalls Stapleton. "But the thing about the '84 game is, he wasn't fit.
"When he got the ball things happened and people did back off him, when you have that aura about you.
"I watched the documentary last week and you saw how the defenders in Italy struggled to cope. When you played against Maradona you knew if you did that, if you weren't close enough to him to stop him passing, he would do you damage.
"He didn't need the extra touch that other players did, he'd deliver that pass with his first touch. There are very few players you come up against in a career who have that aura. And he had it."
On a dreadful pitch, Bryan Robson scored twice to make it 2-2 on aggregate, but on 53 minutes, when Norman Whiteside flicked on a header, Stapleton was unmarked in the box to score what proved to be the winner.
"It's the one goal that people always ask me about but I have scored better ones," he recalls. "And to be on the side which knocked out Maradona was nice."
Frank Stapleton: ‘Maradona had a special aura, so to be on the side that knocked out his Barca was nice’. Photo: Sportsfile
And so the four clubs, and the large Irish contingent, marched into the last four. Liverpool squeezed past Dinamo Bucharest in the semi-final and would beat Roma in the European Cup final on penalties, where the trio of Whelan, Lawrenson and Robinson all won medals.
Spurs beat Hajduk Split in the semi-final and defeated Anderlecht on penalties in the Uefa Cup final with Galvin and Hughton winning medals.
United were drawn against Giovanni Trapattoni's Juventus in the Cup Winners' Cup semi-final, but injuries took their toll, as Stapleton recalls.
There was also regret as Paolo Rossi was left unmarked to score the crucial goal late in the 90th minute of the second leg in Turin.
"He's a World Cup winner, lethal, you can't give someone like Rossi a chance," he says.
Walsh had hoped to get drawn with United in the semi-final, instead they were paired with Cup Winners' Cup holders Aberdeen, and the Portuguese club won both legs against Alex Ferguson's outfit.
But Porto could not outwit Juve in the Cup Winners' Cup final with Walsh coming on as a second-half sub in a final played in Basel. Walsh still queries some refereeing decisions to this day.
Zbigniew Boniek came up with the winner.
"They had some side, Trapattoni was the manager.
"They had Tardelli, Rossi, Platini, Boniek. But we were decent too," Walsh says.
Qualification for a major tournament did not come the way of those players at international level, so they were sustained by club success.
"We were all playing at a very high level and that's the difference between my generation and the Irish national team now," says Stapleton.
"When you play for a club team who are doing well no one underestimates you, and we did prove ourselves at a very good level in Europe," added the Dubliner, who feels that Whelan was never given due credit.
"Ronnie was one of the top midfield players in Europe at the time, when it was very hard to win things," he says.
"We were on a level with all these top players in Europe," says Walsh, now 65 and back to full health after a stroke two years ago.
"I won back-to-back championships with Porto, got to a European final.
"I won seven medals in Portugal but I only got credit in Ireland, not in England.
"Same with Liam Brady... what he was doing in Italy was being reported on and heralded in Ireland, but not in England.
"Anyone going to those leagues from Ireland or England now would be under the constant spotlight, we just did what we had to do. And we didn't do too bad."
Stapleton is pleased with this jaunt down memory lane.
"Going back to the night I scored against Barca, I probably wasn't aware on the night that Mickey had scored for Porto and Ronnie scored two in Lisbon, this was pre-internet," adds Stapleton.
"But three Irish lads scoring to help win the quarter-final of a European cup, I doubt we'll see that again," he concludes.