It's AumSum Time.
What if all the mountains disappeared?
AumSum clones would take their places.
Naahhh.
Earth's solid surface is made up of several tectonic plates.
Mountains are formed when two tectonic plates smash into each other.
Now, if mountains disappeared, then tectonic plates would also have disappeared.
With the tectonic plates gone, hot mantle below would take its place.
Thus making it impossible for us to even walk.
With no mountains, the lava present in them would flow and spread to the surrounding cities.
Global ecosystem would also get disturbed.
As mountains possess a great variety of flora and fauna.
As glaciers on mountains are a source of water for many rivers.
No mountains means those rivers would dry out.
Also, since mountains change the direction of winds and block rain clouds.
There would be catastrophic changes in global weather patterns.
Lastly, without mountains.
We would not be able to paraglide, ski or enjoy river rafting.