Final answer:
Radon gas is the naturally occurring substance that seeps up from the ground and is potentially toxic to people. It is a radioactive gas produced from the natural decay of uranium or thorium found in soil and rocks, and long-term exposure can cause lung cancer.
Step-by-step explanation:
The naturally occurring substance that seeps up from the ground and is potentially toxic to people is radon gas. Radon is a radioactive gas that comes from the natural decay of uranium or thorium found in soil and rocks. It can enter homes through cracks in floors, walls, and foundations, and long-term exposure to its radioactive particles can lead to lung cancer.