Groundwater pollution has become a serious environmental problem.

We should recycle to prevent environmental pollution.

The underground water in this area is very clean and is used as drinking water.

This polluted water could potentially harm our health.

Do you understand the seriousness of this problem?

This problem is very serious.

We must strive to protect the environment of the Earth.

I commute by subway.

The subway is running underground.

The pollution in this area has become a serious problem.