Rewrite the following sentences using the impersonal passive

V. Rewrite the following sentences using the impersonal passive.
1. Everyone believe Son Duong Cave is one of the largest caves in the word . 
2. They estimated one million foreign tourists visited Vietnam in December 2015.
3. We have understood that protecting natural wonders has many benefits.
4. People hope that this complex of monuments will be open to the public soon.