By Alex Kirby
BBC News website environment correspondent
Natural disasters can be a threat to the growing expansion of big cities underground, the United Nations says.
Unsuspected perils may lurk beneath the ground
It says developers often burrow beneath the surface without knowing enough of the risks and with inadequate plans to lessen the effects of any disaster.
The warning comes from experts at the United Nations University just before a conference on how to reduce disasters.
The international conference, which is being held in the Japanese city of Kobe, will run from 18 to 22 January.
The UNU experts say growing urban land pressure is making it increasingly attractive to find new subterranean space for subways, shopping malls, car parks and other needs.
But Dr Srikantha Herath of the UNU says studies of potential natural disaster risks are often neglected.
He said: "The concentration of people and wealth in such underground spaces is expanding and merits careful examination.
"Such facilities in many areas have not been used sufficiently long to be exposed to various types of extreme hazard events of low frequencies.
"Modelling a variety of catastrophic events is essential for building contingencies into underground infrastructure designs, including evacuations and the emergency containment and transport of flood waters, for example."
The university says water can travel long distances underground from a flood source.
But often there are no sub-surface maps, because underground space is usually mapped in relation to a building overhead.
Answers to hand
Dr Janos Bogardi is director of UNU's new Institute for Environment and Human Security, in the German city of Bonn.
He said: "Underground spaces should be designed to withstand multi-hazards.
"For example, fire risks require planners to also include the ability to seal off and compartmentalise underground space quickly."
And underground disasters could also endanger people in buildings overhead.
Dr Bogardi said many infrastructure solutions existed to reduce the magnitude of hazards.
Breakwaters, for example, spared the Indian port of Chennai (formerly Madras) from the worst effects of the Asian tsunami.
"The frequency of underground flooding events is surprisingly high in places," said Dr Herath.
The 1995 Kobe earthquake killed more than 6,000 people
Tokyo records, for example, showed 17 incidents from 1999 to 2001, some involving fatalities despite extensive precautions.
Most floods occurred in the rain and typhoon months, July to September.
Many other coastal mega-cities were vulnerable to floods if certain conditions coincided, such as a heavy rainfall combined with tides or sea surges caused by winds or earthquakes.
Gauging the risk
The UNU is collaborating on a computerised system to simulate the impact of natural disasters such as floods and tsunamis on urban centres, beginning in Asia.
A model of a tsunami reaching the Japanese city of Osawe simulates the evacuation of a coastal population given advance warning.
Case studies of possible tsunamis in other major Asian cities will be presented at the Kobe conference.
Hans van Ginkel, rector of the UNU, said: "Human existence was, and will always be, threatened by hazards of natural and man-made origin.
"Thus human security can be defined better as 'knowing risks' rather than 'eliminating risks'."
The World Conference on Disaster Reduction is being held in Kobe to mark the 10th anniversary of the earthquake which caused widespread damage to the city.