Five key facts about Alcatraz prison

Situated on a tiny island off San Francisco, Alcatraz prison for years held high-profile criminals -- most famously mob boss Al Capone -- before it was decommissioned and turned into a popular tourist attraction.

Here are five things to know about the notorious California jail that US President Donald Trump wants to see brought back into operation.

- Isolated -

The prison sits on Alcatraz Island, a rocky slab of just 22 acres in San Francisco Bay, two kilometers (1.25 miles) from the mainland.

The jail, which closed its doors in 1963 after 29 years in operation, is flanked by cold water with strong currents.

It was rumored to be surrounded by man-eating sharks, but the sharks that live in the bay are actually only bottom-feeders.

- Infamous inmates -

Several notorious criminals were held at Alcatraz after it was transformed in 1934 from a military jail to a maximum-security federal prison.

They included Al Capone -- the Chicago crime boss of the Prohibition era -- as well as George "Machine-Gun" Kelly and Alvin Karpis, once declared "Public Enemy No. 1" by the FBI.

- Escape attempts -

From 1934 to 1963, a total of 36 men were involved in 14 separate escape attempts, according to the Bureau of Prisons (BOP).

In 1962, three inmates fled the prison after they put paper-mache model heads into their beds and broke out through ventilation ducts before leaving the island on a raft.

Their fate remains a mystery, and the getaway was recounted in the 1979 film "Escape from Alcatraz," starring Clint Eastwood as the ringleader Frank Morris.

- Expensive operation -

Alcatraz was almost three times more expensive than any other federal prison due to its remote location, according to the BOP.

Costly necessities included food shipments to the island and the weekly delivery of one million gallons (3.8 million liters) of drinking water.

It never reached its capacity of 336 inmates and at any given time held less than one percent of the federal prison population, the BOP says.

- Tourist hotspot -

After it shuttered in the 1960s due to operating costs, Alcatraz Island became part of the National Parks network and opened to the public in 1973.

More than a million people visit the island each year, with many drawn to exploring its morbid past.