Simplify the following text for a Grade 9 native English-speaking person.
Original: They occurred under water , at the edge of Norway 's continental shelf ( Storegga is Norwegian for the `` Great Edge '' ) , in the Norwegian Sea , 100 km north-west of the MÃ re coast , causing a very large tsunami in the North Atlantic Ocean .
Simplified: They occurred under water , at the edge of Norway 's continental shelf , in the Norwegian Sea , 100 km north-west of the MÃ re coast .