German archaeologist Dr Heinrich Kusch said evidence of the tunnels has been found under hundreds of Neolithic settlements all over the continent.
In his book - Secrets Of The Underground Door To An Ancient World - he claims the fact that so many have survived after 12,000 years shows that the original tunnel network must have been enormous.
'In Bavaria in Germany alone we have found 700metres of these underground tunnel networks. In Styria in Austria we have found 350metres,' he said.
'Across Europe there were thousands of them - from the north in Scotland down to the Mediterranean.
'Most are not much larger than big wormholes - just 70cm wide - just wide enough for a person to wriggle along but nothing else.
'They are interspersed with nooks, at some places it's larger and there is seating, or storage chambers and rooms.
'They do not all link up but taken together it is a massive underground network.' Read More