Have you ever spotted this shape on world maps? You will now ...
It’s a sheer sided valley in the Sudan 3 x 3 degrees across and over 150 m deep !

Fly over it with the ERS-1 altimeter and you only see flat desert.

Comparing the GDEMs with the altimeter heights shows that the whole feature does not exist.

Images: DEM team at De Montfort university, Leicester(PI: Philippa Berry)