One of the alleged 21 July bombers must have been "very, very sure" that a device he claimed to have tested in a flat would not explode, a court heard.
Muktar Ibrahim has claimed the bombings were a hoax
Expert Hans Michels said Muktar Ibrahim's story of detonating 2kg (4.5lb) of main charge to check no-one would be hurt was "amazing".
Prof Michels also told Woolwich Crown Court the devices used on 21 July, 2005, were not capable of exploding.
Six men all deny conspiracy to murder and to cause explosions.
They are accused of attacking London's transport network on 21 July 2005.
Mr Ibrahim, 29, from Stoke Newington, north London, is the first of the six to appear in the witness box.
He has claimed he wanted to test the device indoors to make sure no-one would be hurt.
'Wanted to survive'
Mr Ibrahim has admitted making the homemade hydrogen peroxide and chapatti flour devices, but insisted none of them was capable of causing injury.
The London court was earlier told that he attempted to detonate the charge at the flat in New Southgate, north London, to make sure of this.
Asked for his views of this claim, Hans Michels, professor of safety engineering at Imperial College London, said: "If he tested something which had 2kg of explosives and he wanted to survive that, he must have been very, very sure that it wasn't going off."
The scientist, who is appearing for Mr Ibrahim's defence, added: "That's certainly not something that I would recommend doing.
"If that's your first test, then I think that's amazing.
"I don't quite understand why he would want to do that.
"Certainly I would move."
Prof Michels also told the jury the devices used on 21 July were not capable of exploding.
The detonators were viable, he said, but not powerful enough to set off the main charge.
But he could not come to a conclusion about whether or not the devices were intended to go off.
A presentation by Prof Michels which was shown to the court read: "Is there any conclusive scientific or technical evidence related to an intentional major explosive objective? None that I have been able to identify."
Stephen Kamlish QC, on behalf of Mr Ibrahim's co-accused Manfu Kwaku Asiedu, asked Prof Michels: "It's the main question in this case, do you understand?"
Prof Michels answered: "I do understand."
Mr Kamlish replied: "And you can't answer it."
Mr Ibrahim, of Stoke Newington, north London, is on trial with Mr Asiedu, 33 and Hussein Osman, 28, both of no fixed address; Yassin Omar, 26, of New Southgate, north London; Ramzi Mohammed, 25, of North Kensington, west London; and Adel Yahya, 24, of Tottenham, north London.
All deny charges of conspiracy to murder and conspiracy to cause explosions likely to endanger life.
The trial continues.