Draw a straight line.

He is making music in the basement.

He was locked up in the dungeon.

I commute by subway every day.

I bought fresh fruits in the basement of the department store.

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

He was staring at the horizon.

He was staring at the horizon on the sea.

It is reported that North Korea has conducted another underground nuclear test.

Groundwater pollution has become a serious environmental problem.