What is the antonym for landform?

There isn't a perfect antonym for "landform" in the sense of a single word. However, depending on the context, you could consider these options:

General:

* Water body: This is a broad term for any body of water, like oceans, lakes, rivers, etc.

* Featureless: This describes a lack of any distinctive or prominent landforms.

Specific:

* Ocean floor: This refers to the underwater landscape.

* Atmosphere: This encompasses the layers of gases surrounding the Earth.

* Space: This could be used in the context of comparing Earth's landforms to those on other planets or celestial bodies.

The most accurate antonym will depend on the specific context in which you are using the word "landform."