Workers in the Piccadilly Line tunnel trying to recover bodies are facing temperatures of 60C.
There are still an unknown number of bodies - previously believed to have been around 20 - inside the carriage of the train that was hit by one of Thursday's terrorist bombs.
The tunnel, near Russell Square in London's Bloomsbury district, remains unsafe in the immediate area around the blast.
Engineers are concerned that the crucial steel lining that strengthens the tunnel, which has been bored through clay, may have been ruptured in the blast.
They are considering a plan to drag several carriages down the track to obtain access to the wrecked first carriage in the train where the bomb went off.
Yesterday, the shock waves continued. Above ground, relatives of the missing toured hospitals, asking if John or Jamie or Shahara Islam were there.
They posted messages on websites: have you seen Anthony Fatayi, Behnaz Mozakka, Monika Suchocka, or Ania Brandt? Where is Philip Russell, Susan Levy, Miriam Hyman, Emily Jenkins, Ciaran Cassidy, Neetu Jain, Richard Ellery, and Christian Njoya Small?
And they stuck home-made posters to lamp-posts and walls and phone boxes, appealing for news of their loved ones, many bearing family snaps showing faces almost too carefree to bear.
Above one, carrying a grainy photo of Gordon, a 30-year-old financial adviser, were the words: "Have you seen this man?"
The answer, as many relatives were beginning to acknowledge, almost certainly lay far below in the Underground's tunnels.
In the section near Russell Square, rescuers reported rats, extreme heat, dust and the risk of asbestos - and a still unknown number of bodies.
Maybe 21, maybe many more, lie inside the wrecked carriages.
The bodies - and body parts - will be taken to a temporary mortuary at an undisclosed military site in central London.
Above ground, more yearning. Does anybody know of Karolina Gluck, Benedetta Ciaccia, Helen Jones, Adrian Johnson, Xavier Rebergue?
And what of my sister Laura, asked David Webb, who stood outside King's Cross Underground station yesterday clutching a framed photograph of his 29-year-old sister, who took the Tube to work and has not been seen since.
There are still bodies at the two other Tube blast sites - at Aldgate station and Edgware Rd - although it is thought the Russell Square site is the only one where they remain on the train.
There, the affected carriage is several hundred yards from the station, which is deep underground, around 30m below the surface.
Only a certain number of rescuers can work inside the tunnel at any one time and the teams have to return to the surface periodically, so harrowing are the conditions.
Above ground, the air may be easier to breathe, but the outrage is still as hard as ever to swallow.
Where, for instance, is Anat Rosenberg? An Israeli woman, she was in London because she was terrified to return to her home country because of suicide bombers.
She was talking to her boyfriend John Faulding while on the Tavistock Square bus on Thursday morning.
"At that moment, I heard in the distance horrendous screams and her phone went dead."
He has not heard from her since.
For people such as Mr Faulding, a 24-hour support centre for families of victims of the attacks was set up yesterday in Victoria. Staffed by British Red Cross, Victim Support, Salvation Army, Cruse Bereavement and social services, it is being run by Westminster Council with support from the Metropolitan Police.
- INDEPENDENT, REUTERS