The highest thing on Earth, depending on how you define "thing," is:

* Mount Everest: At 8,848.86 meters (29,031.7 feet) above sea level, it's the highest mountain on Earth, measured from sea level.

However, if we consider things that go beyond the surface of the Earth, then:

* The summit of Mount Everest (relative to Earth's center): The summit of Mount Everest is actually further from the center of the Earth than the highest point on the Earth's surface, due to the Earth's equatorial bulge. This point is about 1.5 meters (4.9 feet) higher than the summit of Mount Everest.

Ultimately, the answer depends on your definition of "highest" and "thing."