Complex sentence: After the mine was closed only a small portion was left open as a tourist attraction and as such the rest of the mine is now flooded due to the pumps no longer being used and is now inaccessible.
Split into simpler sentences: After the mine was closed only a small portion of it remained open as a tourist attraction. As a result, the massive amount of now unused mining tunnels have since become flooded due to the pumps no longer being used and thus, is now inaccessible.