Should Odyssey return treasure to Spain?

Odyssey Marine Exploration inc. located a shipwreck in the Atlantic and recovered over 17 tonnes of gold and silver. The treasure is believed to have come from a Spanish vessel the "Mercedes." Now Spain is demanding that Odyssey return all of the treasure. Odyssey argues that the two century old shipwreck had been abandoned. Odyssey also spent millions in the search and recovery of the wreck. The treasure is estimated to be worth close to 500 million dollars. What do you think should happen?