An Elizabethan padlock has been opened for the first time in more than 400 years after researchers used X-ray imaging techniques to crack its four-letter combination.

The brass lock was part of the Ann Francis treasure hoard, named after the British merchant ship that wrecked during a storm off the coast of Wales in 1583. Researchers believe the lock once secured a chest aboard the ship, before becoming separated from whatever riches it was protecting. Some of the Ann Francis’ cargo was recovered at the time of the wreck, while the rest was claimed by the sea. In recent decades, dredging and storms on Margam beach in South Wales have exposed some of the ship’s lost treasure.

Between 1996 and 2017, metal detectorist Peter Hughes found the lock and much of the hoard, which included gold and silver coins. The lock was subsequently acquired by Amgueddfa Cymru — National Museum Wales, which worked with researchers at Cardiff University in Wales to decipher its combination.

To study the lock’s internal mechanisms, the team used micro-CT scans, which make thousands of X-rays that can then be configured into a 3D digital image. By matching internal notches to external letters in the digital 3D image, the researchers were able to decipher the correct four-letter code — “ANCE” — and open the lock. “We normally use this technology to study bones and other biological structures, so investigating a 400-year-old padlock was certainly a first for us,” David Williams, a research associate at the Musculoskeletal Biomechanics Research Facility at Cardiff University, said in a statement. “It’s not every day you get to solve a 400-year-old mystery!”

The Ann Francis was owned by Francis Shaxton, a merchant and notorious smuggler. Francis claimed the ship was sailing along the eastern English coast from King’s Lynn to Hartlepool, when it was actually traveling in the opposite direction, heading to Spain to deliver a cargo of wheat. Spain was dependent on grain shipments at the time and paid for such shipments in gold and silver from colonial conquests in Central and South America, according to Amgueddfa Cymru.