You can see hotspots by each person in some of the pictures, along with other hotspots to the sides. Likely each person had a remote-controlled slave flash, and

You can see hotspots by each person in some of the pictures, along with other hotspots to the sides. Likely each person had a remote-controlled slave flash, and they positioned other such units for fill. In particular, note the shot of the row of people under the low ceiling. Each person has a hotspot that illuminates the ceiling and silhouettes them; the person is in between the bulb and the camera. In other words, the lights are all oriented the same way, which suggests to me they were set for the shot, rather than being the chance arrangement of the cavers' utility lights at that moment. They might have used long exposures, but the people aren't blurred, so the exposures can't have been all that long. Then again, flash would probably be murder on the eyes for cavers, so maybe these were less-bright-than-flash lamps, with a slightly-long exposure.