The main feature ( the `` Monument '' proper ) is a rectangular formation measuring about 150 by 40 m ( 490 by 130 ft ) and about 27 m ( 90 ft ) tall ; the top is about 5 m ( 16 ft ) below sea level .
The main feature ( the `` Monument '' proper ) is a rectangular formation measuring about 150 by 40 m ( 490 by 130 ft ) , and about 27 m ( 90 ft ) tall .