Earlier this summer, two other pieces of the tunnel were immersed into the water and attached to the tunnels in the rock. This last tunnel shall fit in between the two others, and, as there is not much room for manoeuvre, it is important to get it right.
"We have (a margin of) one metre on each side, but when it gets into place, it is just a question of centimetres, so you really need to make it right," says Swedish Transport Administration press officer Jenny Ryderstedt to Radio Sweden.
" To know for sure that we get it right, we have GPS systems and then we have divers that several times goes down to make sure everything is what is seems to be on the computers and monitors."
The 100 metres long piece of tunnel is the last in the jigsaw to finish this part of Citybanan, which will move the Stockholm commuter trains off the track where the mainline trains go in and out of the capital - doubling the capacity and making the trains less vulnerable to delays. Construction started four and a half years ago and is expected to finish in 2017.
And though there did not seem to be much visible to the eye, as the process on the Tuesday was so slow and gradual, quite a few people had gathered to see it.
"It is the last chance to see the tunnel. And I will be travelling in it.This is my city. I am excited to see it ," said one lady watching the scene.
Stig Bernholm came here at seven o'clock this morning to see the process. He has already been here to watch the two other tunnel pieces being lowered into the water. He admits that there is not so much to see, but he still thinks it is worthwhile to come and watch.
"It is part of the history of Stockholm," he says.
One day he will travel in a train in this tunnel. But before that he hopes that there will be a running competition in the tunnel. And though he is retired, he would definitely want to take part in such a competition.