The National Museum of Underwater Archaeology in the port of Cartagena offers free entry until September 30.

THIS AMAZING MUSEUM, THE FIRST underwater archaeology museum in Spain, is housed in two buildings right on the harbour in Cartagena, directly opposite where the cruise ships dock.

The two buildings for an impressive modern structure, linked below ground level and designed to create an underwater effect. Inside, this remarkable space contains exhibits explaining the fascinating process of underwater archaeology itself, and an eclectic collection of artefacts relating to historic ships used in the ancient Mediterranean maritime trade.

The particular emphasis is on the Phoenicians and Romans, though it displays artefacts from medieval and more modern wrecks. Hanging from the ceiling is a mock up of the framework of a Roman boat, and also on display is a fascinating reconstruction of an ancient Phoenician ship discovered just down the coast at Puerto de Mazarron.

Some of the best features are the reconstructed cross sections of ships showing how the vessels were constructed and how cargoes were stowed. There are some very detailed miniature dioramas showing scenes of port activity and model vessels from various eras.

The last display is about the Spanish frigate Nuestra Senora de las Mercedes, which sank off Portugal in 1804. The wreck spawned an important legal case between the Spanish government and the treasure hunting company Odyssey Marine Exploration, which resulted in the return of about $500 million worth of plundered artefacts to Spain, including a wealth of traditional Spanish coins that are on display at the  museum.