Thoreau goes out to the frozen ice, and chops open a small square. Through the hole, he can see fish, mostly pickerel, swimming around below. Brrr.
Next up is the task of determining how deep the pond is in various spots. He discovers that it's the deepest where the line of greatest length (where the pond is longest) and the line of greatest breadth (where the pond is widest) intersect. He wonders if a man's character can be measured the same way. Analyze that.
Some ice cutters believe that there is a leach hole (basically a drain) at the bottom of the pond that leads to a nearby meadow.
Thoreau also observes how the ice changes its structure as the environment around it changes, too.
In a shocking turn of events, Thoreau observes more. This time, it's the ice cutters, who cut blocks of ice from Walden Pond to ship around the world, even as far as India. This was before FedEx.