Passage: After his investigations Perring buried the robbers tunnel which remains inaccessible to this day. The pyramid of Userkaf entered the official records a few years later in 1842 when Karl Richard Lepsius catalogued it in his list of pyramids under number XXXI. Since Perring had already buried the robbers tunnel by that time, K. R. Lepsius did not investigate the pyramid any further.The pyramid was then neglected until October 1927, when Cecil Mallaby Firth and the architect Jean-Philippe Lauer started excavating there. During the first season of excavation, Firth and Lauer cleared the south side of the pyramid area, discovering Userkaf's mortuary temple and tombs of the much later Saite period. The following year, Firth and Lauer uncovered a limestone relief slab and a colossal red granite head of Userkaf, thus determining that he was the pyramid owner. After Firth's death in 1931 no excavations took place on site until they were resumed by Lauer in 1948. Lauer worked there until 1955, re-clearing and re-planning the mortuary temple and investigating the eastern side of the pyramid. Research on the north and west sides of the mortuary complex was conducted starting in 1976 by Ahmed el-Khouli who excavated and restored the pyramid entrance. The entrance was, however, buried under rubble in an earthquake in 1991. More recent work on the pyramid was undertaken by Audran Labrousse in 2000.

Question: What part of Perring's investigations is no longer accessible?
the robbers tunnel