What does "-4,257m" mean in Google Earth?

When I move my pointer around the pin (89.99 N) shown below, Google Earth displays the camera distance, latitude, longitude accordingly.

I'd just like to know what does "-4,257m" mean, pointed out by the red arrow.

Could someone give me a hint?

• For completeness sake : since , is used as decimal separator in some countries, the , above is used as a thousand separator. So the selected point is 4.2km deep, not just 4.2m. Jan 26 at 8:16