Son Doong Cave.

Short documentaries featuring Son Doong Cave – the largest known cave passage cross-section in the world since 2009 – and other caves in Quang Binh were also screened to introduce the attractions to visitors.
According to Telegraph, newspaper from UK, Son Doong Cave was only explored properly in 2009, with the help of the British Caving Association, and visitor numbers are limited to only 500 people a year. Oxalis, a Vietnamese adventure travel company, is the only operator to offer trips into the caves. It’s 2017 cave tours sold out within 20 hours last summer.
Son Doong lies beneath Phong Nha-Kẻ Bàng National Park, where an influx of tourism could give a much-needed boost to the fledgling local economy. In addition, accordingly, Averie Timm, managing editor at LA’s RYOT journalism agency, helped make a documentary about the cave and what makes it unique in February.
“Because of the limited amount of permits that are granted per year, only our photographer - Jason Speth - was able to enter the cave,” she said. “The whole area is incredible, there's really nowhere in the world quite like it.”
In addition, speaking at the occasion, Nguyen Huu Hoai, Chairman of the Quang Binh Provincial People's Committee said that the festival aims to promote the beauty of the network of caves in Quang Binh Province, which is famous for being home to over 300 mysterious and breathtaking caves, as well as introduce cultural and spiritual values of the stunning tourist site to local and international tourists with the goal of strengthening tourism development in the province.
The ceremony featured colorful art and music performances. The audience was treated to performances showcasing the mystifying beauty of Phong Nha and the local traditional Macoong drums, as well as a play re-enacting the life of late General Vo Nguyen Giap, one of Vietnam’s greatest military strategists born in Quang Binh.
Earlier, the Tourism Association of Quang Binh province opened the Quang Binh Sailing Festival 2017 at Nhat Le Park, Dong Hoi city, on June 16, as part of activities under the framework of the 2017 Quang Binh Cave Festival. The sailing festival, which is scheduled to run until the end of August, features many activities, such as a parade, a sailing competition and a kayaking competition.