Rats, mice, dust, stale air and the stench of decomposing bodies were all contributing to the hellish conditions in the tunnel 500 metres from Russell Square station, police said.
It could take at least two days to recover the unknown number of bodies trapped in the wreckage of the Piccadilly Line train. Twenty-one people are known to have died in that blast, but the front carriage, where the explosion happened, has yet to be reached and is thought to contain many more bodies. There is no hope of finding anyone alive.
Only a certain number of people can work inside the tunnel at any one time and the teams have to return to the surface periodically, so harrowing are the conditions, the police added.
Scotland Yard's senior identification manager, Detective Superintendent Jim Dickie, said the extreme heat was a "significant factor" in the recovery operation close to Russell Square Tube station.
There are still bodies at the two other Tube blast sites - at Aldgate station and Edgware Road - although it is thought the Russell Square site is the only one where they remain in situ on the train.
Deputy Chief Constable Andy Trotter, of the British Transport Police, warned that it would be some time before the "methodical and meticulous" recovery operation was finished.
"The search was halted overnight because of the increasingly difficult conditions but it resumed again early this morning," he said yesterday. "We have had to stop and regroup, look at the risks and dangers and proceed much more cautiously.
"I have visited the search teams and they are facing particularly difficult conditions. It is extremely hot, very dusty and quite dangerous down there."
He continued: "The officers will be using a variety of cutting equipment to try to prise the fabric of the train apart."
Refrigeration units have been taken to the scene to store the bodies - and body parts - before they are taken to a temporary mortuary at an undisclosed military site in central London.
The tunnel, built more than 100 years ago, is one of the deepest on the entire Underground network. Its narrowness has made it impossible to remove bodies through the side doors or to take cutting equipment alongside.
DCC Trotter added: "As you can imagine, on a packed train at rush hour in a very tight tunnel, the damage is considerable. The officers are highly experienced and are cautiously making their way through. They are recovering bodies throughout the day."
Rescue workers are working alongside forensic experts from Scotland Yard, who are desperately hunting for clues about the type of explosives used in the bombs and the identity of the bombers.
"We have to work on the assumption that those who did this are still out there and that they could do it again," the security source said. "Potentially, the forensic work there could save lives in the future."
Forensic experts were also poring over the wreckage of the number 30 bus that was destroyed in Tavistock Square, close to Russell Square station, in the search for clues. The roof of the bus, which was blown off, has been taken away to be examined. The rest of the vehicle is likely to be removed today.
Once the bodies have been recovered and the train wreckage removed from the Piccadilly Line, the tunnel will have to be repaired. London Underground does not expect to open that stretch of line for several weeks.
A TRAGEDY UNFOLDS
Police and transport authorities yesterday gave their most detailed account of events:
8.50am: Three bombs explode within a minute on London Underground trains.
8.51: Police get their first call to the Aldgate explosion.
9.15: All trains are stopped by London Underground. Press Association reports emergency services have been called to an explosion at Liverpool Street.
9.17: Police get first call about Edgware Road. All Tube stations are evacuated.
9.33: London Underground says there has been "another incident at Edgware Road" station.
9.40: British Transport Police say five "power surge incidents" - some causing explosions - have occurred on the Underground.
9.47: Thirteen people die when an explosion tears through the back of the no 30 bus.
10.02: Scotland Yard says its officers are assisting with a "major incident".
10.10: It is confirmed that Tony Blair at the G8 Summit in Gleneagles is being updated on events.
10:28: London Fire Brigade says it has been called to four "explosions".
10:39: All London hospitals are put on major incident alert.
11:00: Members of the public are told not travel to London.
11:16: Metropolitan Police Commissioner Sir Ian Blair says there have been at least six explosions, but claims the picture is still "very confused".
11:24: Mobile phone operators confirm networks are jmed.
11:48: The FTSE 100 Index plunges more than 200 points.
11.52: All London hospitals are full, police confirm.
12 noon: Tony Blair addresses the nation and says an unknown number of people have died in a series of "barbaric" terrorist attacks.
12:46: Pope Benedict XVI calls the attacks "inhuman".
12:57: "Secret Organisation Group of al-Qa'ida of Jihad Organisation in Europe" claims responsibility for the London attacks on an Islic website.
13:08: G8 leaders condemn the "barbaric" attacks on London.
13:12: The Mayor of London, Ken Livingstone, says the city is the target of a "cowardly terrorist attack".
14:16: The Muslim Council of Britain says it "utterly condemns the perpetrators of what appears to be a series of coordinated attacks".
16:06: The Archbishop of Canterbury, Dr Rowan Williams, says: "The appalling events in London this morning have shocked us all."
17:29: The RAC reports that major routes out of London are jammed. Many hotels in the capital are totally booked up.
18:26: Police say the number of confirmed deaths is 37.
FRIDAY: 8:00am: Many resilient commuters return to work on disrupted public transport, but many thousands stay away.
8:59: Home Secretary Charles Clarke says the terror attacks "came out of the blue".
11:24: Sir Ian Blair says around 50 people are now feared dead.
11:44: The Prince of Wales visits victims at St Mary's Hospital in Paddington, paying tribute to the "resilience of the British people".
12:31: "Strong possibility" that an al-Qa'ida-linked group was responsible, Charles Clarke says.
16:00: Transport said to have returned to near normal.
16:09: The Queen gives a defiant message while visiting the Royal London Hospital, saying: "They will not change our way of life."
17:09: At British mosques, imams condemn attacks. Muslims are urged to stay "calm and vigilant" amid fears of reprisals.