We asked them why they thought they were hitting water. There were many theories! Eventually, they also noticed that if they poured water on top of the snow that it would quickly disappear.
Once inside, we did a quick experiment to illustrate what was happening to the snow as it melted and filtered through the layer of granular snow and pooled on top of the frozen sand below.
The children noticed that the water could filter through the layers of marbles and settle at the bottom of the jar. Mystery solved!