Complex sentence: It was first named "Grim's Graves" by the Anglo-Saxons and was first excavated in 1870 when the pits were first identified as mines dug over 5,000 years ago.
Split into simpler sentences: It was first named "Grim's Graves" by the Anglo-Saxons. The site was first excavated in 1870 when the pits were first identified as mines dug over 5,000 years ago.