If the highest part of the land is a mountaintop,

Where is the lowest place on earth?

That’s right—in a valley.
The lowest point on dry land is the shore of the Dead Sea, shared by Israel, Palestine and Jordan, 414 m (1,360 feet) below sea level.

The Valley of the Dead Sea was not made by a river, as many valleys are.
It is a fault—a large strip of the earth’s crust that has sunk down.
Dead Sea by David Shankbone.jpg
"Dead Sea by David Shankbone" by David Shankbone - David Shankbone. Licensed under CC BY-SA 3.0 via Wikimedia Commons.