Simplify the following text for a Grade 4 native English-speaking person.
Original: By around 13,000 years ago the sea level was only 60 m lower than the present day , and corals began to grow around the hills of the coastal plain , which were , by then , continental islands .
Simplified: By 13,000 years ago the sea level was 60 metres lower than the present day . Corals began to grow around the hills of the coastal plain .