It's 7 km from the toll bridge crossing the Nam Song. Most people arrive by tuk-tuk, motorbike or bicycle. The road is flat, but very rocky, so if you come by bicycle, be prepared for a sore butt. We decided to walk. The road is well-marked with signs both in Lao and English, but beware of signs attempting to sidetrack you to other caves. These other caves might be perfectly charming, we didn't have time to investigate, but the signs you want to follow are clearly marked "Blue Lagoon." The road is very dusty, and walkers are low men on the totem pole, so we were pretty ragged by the time we arrived. An entrance fee of 10,000 kip is the price of admission.
The lagoon is indeed very blue. On one side there is a rope tied to a large tree. You can climb the tree or swing on the rope to plop into the lagoon. Nearly every doing this was a Westerner in his/her 20s. On the other side of the lagoon, there are mostly Asians wading in the water and looking with amusement at the fleshy farangs cavorting in their skimpy bathing suits.
Running across the center of the lagoon is a foot bridge leading to a restaurant and the path to the cave. The climb up the mountain is very steep and treacherous, but there are bamboo hand rails throughout. When you reach the cave entrance, you will have had about 20-30 minutes of strenuous aerobic activity. If you didn't bring your own torch, you can rent one for 10,000 kip before you enter the cave. After climbing over some very slippery rocks you reach the center of the large cave with a small reclining Buddha. This part of the cave is bathed with natural light. If you want to explore further you can continue to the other side of the cave. We didn't do that because it was too slippery for us.
For our return trip, we decided to hire a tuk-tuk. There were several parked in the shade near the lagoon. The price was 100,000 kip to return to town, or 80,000 if you get off before the toll bridge.