Complex sentence: In the 1970s the maintenance tunnels connecting the different parts of the building were sealed, however over the years there have been a number of students claiming to have found an as yet undiscovered entrance to the shafts that lead to these tunnels.
Split into simpler sentences: In the 1970s these tunnels connecting the different parts of the building were sealed. However, over the years, there has been a number of students claiming to have found an, as yet undiscovered, entrance to the shafts that lead to these tunnels.