I’m a commercial diver, and was once on a job cleaning a potable water reservoir. I’d been in other reservoirs before, but this was by far the biggest, at 40×80 metres. To get in you had to open a hatch in the ground (the whole reservoir was underground) and climb down a ladder.

The hatch was in a corner, so when you were in the far corner of the reservoir, it was completely pitch black, and you just had to hope your light didn’t go out. I was about half way through a three hour dive when the batteries in my torch started going flat. I watched the beam get narrower and dimmer until it cut out completely.

It’s not a huge problem if you lose light, as you can just follow your umbilical back to the hatch. Just as I started walking back, some obnoxiously loud banging started somewhere in the reservoir. I was the only diver in there, so it both confused and scared the s@#t out of me.

Needless to say I ran back to the hatch as fast as I could. I ended up getting my torch changed out and doing another hour in the water, but didn’t hear the noise again. I still have no idea what it was, but the combination of my torch going out and loud banging coming from somewhere gave me a hell of a fright.