When did Guantanamo Bay open?
The detention site at Guantanamo Bay, Cuba opened on January 11, 2002, when the first inmates from America’s “war on terror" arrived.
They were kept in chain-link cages in a secluded part of the US Naval Station Guantanamo Bay. The original prison, known as Camp X-ray, was assembled in less than 96 hours.
While the prison was first used in 2002, the US has maintained a naval base in Cuba since 1903.
Why did the US choose Guantanamo Bay?
In the months after the US launched the war on terror, its response to the September 11, 2001 attacks that killed nearly 3,000 people, the administration of George W Bush rounded up almost 800 terror suspects.
They needed a place to keep them away from prying eyes and outside of the US, so they would not be entitled to the rights afforded by the US justice system. While several locations were considered, the US settled on Guantanamo Bay, Cuba.
The 117-square-kilometre base had ample space and was not technically part of the US, but rather an "island outside the law".
Was torture used there?
Inmates and human rights groups have reported mistreatment, abuse and torture at the hands of CIA interrogators and guards.
“The Guantanamo Bay military prison is a catastrophic legal, moral and ethical failure," said Hina Shamsi, director of the ACLU’s National Security Project, about the prison's birthday.
"It is a global symbol of American injustice, torture, and abuse of power.
"It’s a national disgrace that our government has for 20 years indefinitely detained Muslim men at Guantanamo, and persists with unconstitutional and secretive military trials.
"President [Joe] Biden needs to finally put an end to this extralegal and abhorrent prison and system."
Mohamedou Ould Slahi spent 14 years at Guantanamo Bay where he was kept in isolation and subjected to sleep deprivation tactics – blasted with heavy-metal music and strobe lights, and drenched in ice water – he told The New York Times.
His experiences at Guantanamo Bay were recently turned into the Hollywood film The Mauritanian.
Lawyers for detainees say their clients have suffered irreparable physical and emotional trauma while at Guantanamo.
Who is being held there?
There are still 39 inmates at Guantanamo Bay. The accused mastermind of the 9/11 attacks, Khalid Sheikh Mohammed, and accomplices are the most high-profile among them. They have been waiting for a military commission trial postponed several times.
High-profile inmates who have been released include Canadian Omar Khadr, who was the youngest suspect held there when he was captured by US troops at the age of 15.
Mr Khadr, who admitted to killing a US soldier during a gunfight, was deported to Canada in 2012.
The war in Afghanistan is over, so why is Guantanamo still open?
President Barack Obama tried and failed to close the prison, going as far as to issue an executive order calling for prison’s closure his third day in office. But after 8 years of Republican obstructionism, his hopes to close the facility were dashed.
President Joe Biden has also pledged to close the prison, but has so far made no meaningful moves.
Since Mr Biden took office only one prisoner, Abdul Latif Nasser, has left the base. Granted a release under the Obama administration in 2016, paperwork problems and former president Donald Trump halting all releases delayed Mr Nasser's return to Morocco for five years.
How much does it cost the US to operate the prison?
It costs the US government $540 million a year to operate the detention centre. That means it is now costing US taxpayers about $13 million for each prisoner, making Guantanamo probably the most expensive prison in the world.
The average cost for each person in prison in the US is between 14,000 and $70,000 Vera says.