Tributes and well-wishes flooded in from people around the world as the 18th year anniversary of the 9/11 terrorist attack was remembered this Wednesday.

Through the powerful statement urging people to #NeverForget, Twitter and various social media sites were filled with sentiments honoring the fallen, the victims, the survivors, the heroes and those that responded and helped⁠—many of whom, up until this day, are still recovering from the psychological trauma and illnesses related to the disaster. It could be recalled that the terror attacks claimed almost 3,000 lives and injured more than 6,000 others.

To this day, it was considered the deadliest attack in world history.

Looking back

It was September 9, 2001, a Tuesday morning, when nineteen men hijacked four American passenger planes. Two of these were crashed into the twin towers of the World Trade Center complex located in Manhattan. After approximately two hours, the buildings collapsed.

The third plane was crashed into the Pentagon, the headquarters of the United States Department of Defense. The fourth plane was intended to hit Washington, D.C. but instead crashed in a field in Shanksville, Pennsylvania after the passengers bravely fought back to the hijackers.

All 265 passengers from the four hijacked planes died that day.

And as of late, even 18 years after the tragedy, CNN reports that only 60% of nearly 2,800 deaths in the World Trade Center were identified.

The fatalities didn't stop until this day, as thousands are still battling traumas, cancers and respiratory illnesses brought by exposure to the toxic dust and debris cloud coming from the collapsed towers.

Al Qaeda leader Osama Bin Laden, who was killed by US Navy Seals in 2011, claimed responsibility for the attacks.

Memorials and Remembrance

In commemoration of the attacks, the US held memorial services all over the country, particularly in New York, Virginia, and Pennsylvania—the three crash sites of the four hijacked airplanes.

At least six 'moments of silence' were observed in the ceremonies, four for when the planes struck the twin towers, the Pentagon, and the field in Pennsylvania; and two for the times when the towers collapsed.

Public schools in New York also partook, following the implementation of a new law that instructs public schools to offer a moment of silence yearly to pay respect to the 9/11 attacks.

President Trump also arrived at the Pentagon for a memorial service and gave a speech for the victims, the survivors, and their families.

Name-reading ceremonies were also held both in the Pentagon and the National September 11 Memorial & Museum; all the names of the identified victims of the 9/11 attacks were solemnly read.

On the other hand, six huge slabs of stones were also placed on the memorial site to silently commemorate and honor the casualties that are yet to be found and identified, and aren't included in the list of names.

The world remembers

Many people from around the globe expressed their well-wishes and remembrance for the 9/11 anniversary and vowed never to forget the horrible disaster. Twitter, particularly, through the hashtag #NeverForget, stood with the victims and their families by posting their thoughts, sentiments, and various media that honored and paid tribute to the tragedy.

In the National Atlantic Treaty Organization HQ, the flags were lowered to a half-mast, or below the summit of a pole, as an act of solidarity. "We stood with you then & stand with you now," tweeted Piers Cazalet, Deputy Spokesperson of the NATO.

Various embassies in the US, such as Germany, Sweden, Turkey, etc.

also brought their flags lower to honor the anniversary.

In a viral video circulating Twitter, firefighters from New Zealand are seen performing a haka to honor the firefighters and first responders who died doing their duties. It could be noted that a Haka is the war dance of the Maori people of New Zealand, usually composing of synchronized movements, stomping, and shouting. There are varying versions, but it mainly revolves around respect, BBC reports.

And though it has been nearly two decades since then, many could still suffer from the anxieties and trauma caused by the disaster.

Some tweets expressed their concerns regarding the posting of pictures of the twin towers on fire. "The families of the thousands killed that day and those of us who witnessed it don't need to see that again in order to #NeverForget," user DubTrixxx posted.

Kiss 107.1, a Cincinnati music station in Ohio, also shared a note regarding the posting of triggering photos.

As of the moment, the hashtag #NeverForget is still trending worldwide with more than 800,000 tweets and counting⁠—mostly filled with heartfelt tributes for the victims of the September 11 tragedy.