One of the challenges with GoPro video is lighting (sound is a bigger one). I just got a Sunpak LED 16 light for the GoPro. It works with the Hero3+ and Hero4 series. I have not yet used it in a cave (only in a dark house). But it throws a heck of a lot of light. The light lasts about two hours, maybe a tad less. There are three lighting levels. Low is hardly better than a friend with a good Zebra Light. Medium and High are good. The box says 300 lumens.
On the upside, the light setup is waterproof and the light levels work to illuminate scenes. There are 3 levels plus a SOS flashing level.
On the downside, it adds about double the size and weight of a basic GoPro and waterproof case. I don't imagine it will be comfortable to have it attached to my helmet for long. It comes with a dual mounting bracket - one side for the GoPro and one for the flash assembly. Setup was easy.
It has to be recharged from a USB link (ie, to a laptop or similar). I have not yet looked at buying a second battery for mine. Has anyone gone that route or got a different work-around?
Any other tips?