The name is a quiet joke of scale. Locals call the Mediterranean the Mar Mayor, the greater sea, which makes this lagoon its lesser cousin, the smaller sea held just inside the coast. It covers about 135 square kilometers yet reaches barely seven meters at its deepest, so its water stays warm, clear and unusually salty, salty enough that swimmers float with almost no effort. A long, thin sandbar called La Manga, "the sleeve," seals it off from the open Mediterranean, running some twenty-two kilometers and narrowing in places to a hundred meters wide. Cape Palos anchors its southeastern corner and gives the lagoon its rough triangular shape. Five low islets break the surface within it, and the calm, shallow water has made it one of Spain's favorite grounds for sailing and other water sports.

People have worked these shores for a very long time. Evidence of human occupation reaches back to the Paleolithic, with Copper Age finds at the Marchamalo salt flats. The Romans knew the lagoon as "Palus" and prized it twice over: as a sheltered harbor for their ships and as a source of the salting industry that produced garum, the pungent fermented fish sauce Rome could not do without. Later, under Arab rule, fishermen built ingenious traps in the natural channels linking the lagoon to the sea. After Alfonso X of Castile took the Taifa of Murcia in the thirteenth century, the Muslim population was expelled and Christian settlers moved in. For centuries afterward the coast lived under the shadow of Barbary corsairs; monarchs including Charles V and Philip II ordered watchtowers and fortifications raised along La Manga and Cape Palos, yet the raids continued, and much of the shoreline stayed thinly settled and wary.

The lagoon's modern life began with its supposed powers of healing. In the nineteenth century towns like San Javier and San Pedro del Pinatar took shape along the water, and by the early twentieth century visitors were arriving for "novenarios," a prescribed cycle of nine therapeutic saltwater baths meant to restore the body. Lighthouses rose, an air-naval base was established at Santiago de la Ribera, and hotels and summer villas spread along the sand. Under the Franco era the transformation accelerated: large-scale investment poured into La Manga, consolidating land and covering the once-empty sandbar with resorts and apartment blocks. The building boom brought money, but it came without environmental safeguards, and it began to change the very chemistry of the lagoon, lowering its salinity and stripping away habitat that had taken millennia to form.

The northern salt flats remain a protected natural park and a wetland of international standing, recognized under the Ramsar Convention in 1994 and prized for its birdlife. But the wider lagoon has been fighting for its life. By 2016, after eighteen years of ignored warnings, pollution had pushed it to the edge of collapse; in 2017 its beaches lost their Blue Flag status entirely. The cause sits in the fields around it. Intensive farming washes nitrates, ammonium and phosphates into the water, feeding explosions of algae and bacteria that strip out the oxygen and leave fish to suffocate, as they did by the thousands in 2019 and again in 2021. Out of that grief came something unprecedented. Beginning in 2018, ordinary citizens organized under the banner "ILP Mar Menor" to demand the lagoon be treated as a legal person, and in October 2022 Spanish lawmakers agreed. The Mar Menor became the first ecosystem in Europe with the legal right to exist, defended by a council of citizens, scientists and officials, on whose behalf anyone at all may now go to court.