Radioactive steam would vent from the mountain and contaminated water would spread underground.

If a fire line is not dug down to mineral soil, then the fire can spread underground through the duff and start in another spot.

What's fascinating to me is looking at how street-level panhandling has spread underground.

There was more debris from the life that had chosen to spread underground.

The gasoline has spread underground over 30 acres and is, at some points, seven feet deep.

Aplectrum hyemale will spread underground through the growth of its tubers, forming large colonies.

There, the fungus can spread underground through root grafts.

Lawn grass plants are stoloniferous, that is they form roots that spread underground.

The hummock roots spread underground beyond the prickly clump and deeply into the soil, forming an immense cone.

The fleshy roots (1-2 cm in diameter) are rhizomes that spread underground.