Kong Lor Cave is an amazing 7.5 km-long limestone cave that was formed by the Hinboun River which still flows through the cave year-round. You can take a boat ride right through the main cavern, which is up to 90 meters wide and 100 meters high.

The Konglor Cave can be reached via a 40 km overland trip from Ban Khoun Kham , the 'Gateway to Konglor' or by taking a slightly longer but more adventurous boat trip up the Hinboun River beginning in beginning in Napouk Village.

Reference:
http://www.ecotourismlaos.com/activities/konglor.htm