As the Athabasca Glacier ice melts, water runs off along the surface. Occasionally, it will drill down with the aid of some fast-moving rocks and grit, and plunge through the glacier until hitting another stream layer (or bedrock) below. These holes are called glacier mills, or moulins. A few people fall into them accidentally, which is a death-sentence. Our guide told us that in some places, well-prepared divers will go into mills on purpose to explore below the surface of millennia-old glacier!