The discovery came as researchers from the US-based Glen Dash Research Foundation and the Ancient Egypt Research Associates (AERA) were studying the magnificent structure to better understand its original dimensions.

Way back when the pyramids were constructed 4,500 years ago, under the rule of Pharaoh Khufu, they were encased in white limestone. Over the years, this limestone outer shell was stripped away – likely for other building projects – leaving the structure we know today.

thanks to sciencealert.com for the great info

thanks to Jorge Lascar for the pic