Complex sentence: S-4 (Sector Four) is an alleged military base said to be near Papoose Lake, supposedly hidden under the Papoose Range within Nevada's Nellis Air Force Range (now referred to as NTTR) at Area 51.
Split into simpler sentences: S-4 (Sector Four) is an alleged military base said to be located near Papoose Lake, Nevada. The facility is alleged to be located underground under the Papoose Range within Nevada's Nellis Air Force Range (now referred to as NTTR) at Area 51.