The first simple fact is that the scrabbling noise from the Dead Room is almost certainly the hundreds or thousands of large black rats feasting on Wilkes’s frozen comrades. The Norway rats – as Crozier knows better than the young Marine – are nocturnal, which means that they’re active day and night during the long arctic winter, and the creatures have teeth which constantly keep growing. This, in turn, means the God-damned vermin have to keep chewing. He has seen them chew through Royal Navy oak barrels, inch-thick tins, and even lead plating. The rats are having no more trouble down there with the frozen remains of Seaman Walker and his five unlucky comrades – including three of Crozier’s finest officers – than a man would have chewing on a strip of frigid salted beef.

But Crozier doesn’t think it’s only the rats that Manson and the others are hearing.

Rats, as Crozier knows from the sad experience of thirteen winters in the ice, tend to eat one’s friends quietly and efficiently, except for their frequent screeching as the blood-maddened and ravenous vermin turn on one another.

It’s something else making the clawing and banging noises down on hold deck.

What Crozier decides not to remind Private Wilkes of is the second simple fact: while the lowest deck would normally be cold but safe there beneath the waterline or winter line of frozen sea ice, the pressure from the ice has forced Terror’s stern more than a dozen feet higher than it should be. The hull there is still locked in, but only by several hundred heaped tons of jagged sea ice and the added tons of snow the men have piled alongside to within a few feet of the railings so as to provide more insulation during the winter.

Something, Francis Crozier suspects, has dug down through these tons of snow and tunneled through the iron-hard slabs of ice to get at the hull of the ship. Somehow the thing has sensed which parts of the interior along the hull, such as the water-storage tanks, are lined with iron, and has found one of the few hollow outside storage areas – the Dead Room – that leads directly into the ship. And now it’s banging and clawing to get in.