The British Gelogological Survey registered Monday's hydrogen explosion at 4.7 on the Richter scale - similar to a light earthquake. But the failure of seismograph stations to register today's explosions highlights some of the problems with a nuclear test ban. The CTBT says that monitoring stations only need to detect explosions from bombs averaging more than 1000 tonnes of explosives, which would only produce a ground 'shake' of 1 nanometre in size. Below this level, noise from other events, such as mudslides, earthquakes or building construction sites interfere with nuclear 'signatures'. Smaller explosions, particular those taking place underground, are therefore hard to observe.

For this reason, the treaty also implements on-site inspections, radionuclide monitoring and acoustic sensors to look for nuclear testing on known test sites. In all cases it is unlikely that underground sub-kiloton yields will leave enough signatures to pinpoint the location of any clandestine explosions outside these areas.

India has so far refused to sign the 1968 Nuclear Non-Proliferation Treaty (NNPT) and the Comprehensive Test Ban Treaty. It claims the treaties discriminate in favour of the the official 'nuclear' club - the US, Russia, UK, France, and China - who can hold onto and refine nuclear arms indefinitely and with no commitment to disarm.

The international response to the news has been vocal, but only the US and Japan, the largest donors of economic aid to India, have introduced sanctions against the country. Pakistan, which is only 93 miles from the test site, has threatened to implement its own nuclear weapons programme if India builds up a nuclear arsenal. The explosions also undermines the NNPT observed by 185 countries.

United Nations Secretary - General Koffi Annan expressed "deep regret" over the blasts and the European Union issued a statement voicing dismay over India's decision. However, non-profit organisations such as Abolition 2000 UK say that India should not get all the blame. "Unless all the nuclear weapon states begin serious multilateral negotiations now, nuclear weapon proliferation is inevitable" they say.