Looking up the rope, tautly umbillicalling me to the darkness of the cave above, I caught myself mentally checking off the safety redundancies for what must've been the third or fourth time since I started the descent: the harness, the knots, the helmet, the rope, the radio, the knife, the proximity alarm, the anchor I'd selected, the backup, and finally Tim; my partner on this mission. As if on cue, his voice, quietened by distance, came from above: "what's the hold up?" "Nothing! I was just making sure we'd not misread the depth. I reckon I'm about halfway down now. Everything's ok, no need to worry." I realised I was blabbering now, as I often did when talking to Tim. Something about him set me on edge. Or maybe… "It's fine. I'm fine. We can keep going. I'm starting again now." I descended another 10 or so feet. "Tim?" A pause. Then the rope started shaking, violently.