Complex sentence: It was designated a UNESCO World Heritage Site in 2000 for its pristine tropical rainforest ecosystem, and contains 16,000 square kilometres (6,178 sq mi) of both montane and lowland primary tropical forest including sections of the Guyana Highlands.
Split into simpler sentences: It was designated a UNESCO World Heritage Site in 2000 for its pristine tropical rainforest ecosystem. It contains of both montane and lowland primary tropical forest including sections of the Guyana Highlands.