It's exactly 30 years to the date when two thieves, who posed as policemen, walked into the Isabella Stewart Gardner Museum in Boston and managed to walk out with half a billion dollars-worth of artwork.

On the morning of March 18, 1990, the thieves successfully managed to escape with 13 pieces of iconic artwork from the likes of Rembrandt van Rijn and Edgar Degas.

Thirty years later, it remains the largest unsolved art heist in history.

ITV News has spoken to Art Critic and University of Arts London mentor Estelle Lovatt, who explains how art thieves are unintelligent and how stolen art has no value to a legitimate buyer.

She said she thought this particular robbery was "extraordinary in terms of the iconic works of art".

Rembrandt van Rijn's 'Christ In The Storm On The Sea Of Galilee'. Credit: Isabella Stewart Gardner Museum

When asked who would buy these types of art, she said: "There would be no legitimate buyer, stolen art goes hand in hand with money laundering.

"People will often pay big money to have artwork they can show off almost like an identity token, to boost your ego if you like.

"But stolen art won't make much as people can't show it off properly."

She said: "Essentially, stolen art becomes worthless, it has no value, nobody would touch it because you can't show them."

Five works on paper by Edgar Degas were stolen from cabinets in the Short Gallery. Credit: Isabella Stewart Gardner Museum

However, Lovatt said if the paintings were handed back in to the gallery or found, the paintings would be "priceless".

She told ITV News: "I really can't put a price on it, the artists are dead so these pieces of work cannot be done again or replicated by the same people."

"You've got the Rembrandt, Degas, these are really unique pieces, they cannot be painted again in the same way."

"You know we've heard this could've been an inside job, it could've been connected to the mafia, we just don't know what could've happened."

"If the FBI can't find them then maybe they won't ever be found."

Art critic Estelle Lovatt in front of artwork by Deborah Azzopardi. Credit: Cristina Schek

Despite the fact that 13 pieces of art were missing Lovatt said the museum is "wonderful" and said it wouldn't affect visitor count.

She said: "The funny thing about missing art is that it doesn't affect visitor numbers to the gallery."

She also compared the robbery to that of the missing Mona Lisa in 1911, which was subsequently returned.

"For instance, when the Mona Lisa was stolen, people queued for ages just to see the missing space," Lovatt said.

Of the more than 35,000 works of art in the Louvre, perhaps none is more popular than the Mona Lisa. Credit: AP

"People will go in to see nothing and talk about what happened in the gallery, it adds a fourth dimension," she added.

In a statement the Isabella Stewart Gardner Museum said despite promising leads in the past, the theft of 1990 "remains unsolved".

It added: "The Museum, the FBI, and the US Attorney's office are still seeking viable leads that could result in safe return of the art.

"The Museum is offering a reward of $10 million for information leading directly to the recovery of all 13 works in good condition."

A separate reward is being offered for the Napoleonic eagle finial. Credit: Isabella Stewart Gardner Museum

"A separate reward of $100,000 is being offered for the return of the Napoleonic eagle finial."

Anyone with information about the stolen artworks or the investigation should contact the Gardner Museum directly.