Around the corner from Ban Gioc Waterfall is Nguom Ngao Cave, one of the biggest caves in Vietnam.
Huyen and I ventured into the cave which used to serve as a hiding place for Vietnamese soldiers when they had a border war with China in the 1970s. The cave was pretty spectacular and unlike other Vietnamese caves (cough cough Halong Bay) isn't cheesily lit up with Christmas lights. There were lots of cool things in the cave but this video shows the upside down lotus flower:
I've got some cool photos from in the cave but alas, my camera wire still isn't working!