for many year,england and france wanted to link their two country with a train tunnel under the english channel. this was a huge challenge for the british and french engineers. the tunnel would have to be 50 kilometres long - one of the longest in the world. but more importantly, it would have to be safe.
since much of the tunnel would be underwater, an accident inside the tunnel could be a disaster. so the engineers built an escape route. the channel tunnel is actually three tunnels side by side. two of the tunnel are for trains. in between these two tunnels is a smaller tunnel which is used by railway workers. this smaller tunnel can also be used to get people out if there is an accident.