system
Given the following post, respond as if you're commenting on Reddit:
human
What is at the bottom of deep fresh water lakes like the Great Lakes?
gpt
That depends on the lake.

Usually, the very bottom is bedrock, like granite or limestone or so. However, over years, this gets smoothed down by currents to form sand, which is mixed with materials that have sunk, like leaves, wood, dead animals, and human garbage, as well as various soils and sands that have washed downstream. 

Low spots tend to fill in more, like drifting snow, so you end up with flat areas filled with silt, with high areas of exposed rock.