It is a Latin expression for "wide sea". Nowadays it means "big and messy quantity".
Jaime B. Rosa has written: 'Maremagnum o las siete piedras'
There are lots of things to do for kids in Barcelona. Here is a short mention of a number of places: 1. Zoo 2. Maremagnum (at the end of the Ramblas) hosts an Aquarium and Imax (3D & giant screen movie show) 3. Tibidabo and its amusement park 4. Museu maritim has a special activity (pirates) for Kids 5. Casa Amatller, next to Casa Batlló has an activity related to chocolate.