Complex sentence: Ha Long Bay has an area of around 1,553km², incuding 1,960 islets, most of which is limestone, the core of the bay has an area of 334km² with highly denstiy of 775 islets.
Split into simpler sentences: Ha Long Bay has an area of around 1,553km², incuding 1,960 islets, most of which are limestone. The core of the bay has an area of 334km² with a high density of 775 islets.