Simplify the following text for a Grade 9 native English-speaking person.
Original: A limited part of the network ( 1.7 km ) has been used as an underground ossuary , known as the catacombs of Paris . The catacombs were temporarily closed between September and 19 December 2009 , after which they could be legally visited again from the entrance on Place Denfert-Rochereau .
Simplified: A limited part of the network ( 1.7 km ) was used as an underground ossuary ( storage place for human bones ) in the past known as the catacombs of Paris , and can be legally visited from the entrance on Place Denfert-Rochereau .