A chemical reaction in an unmarked cesspit caused an explosion as work on the BioMalta life sciences centre in San Ġwann was under way yesterday.

No one was injured and the site was cordoned off for testing, with experts giving the all-clear for work to continue some distance away from the explosion site.

An excavator operated by 37-year-old James Bonavia from Xemxija uncovered the cesspit at about noon, causing a strong chemical reaction.

“I was operating the machine as I do every day, collecting debris and, all of a sudden, there was an explosion and I saw a thick, yellowish plume,” Mr Bonavia, the managing partner of 3Bees Joint Venture Construction Company Ltd, said soon afterwards.

Stones went flying and one large brick penetrated the temporary boundary wall around the site.

Workers who were on site during the explosion said the stone had missed a woman walking on the pavement by hardly more than just a metre.

Mr Bonavia said he considered himself lucky to be alive and that no one was injured, adding that no damage was done to the machinery either.

The explosion halted excavation work on the former Malta National Laboratories site, which is making way for the new biopark project, partly funded by European Union funds.

Civil Protection Department personnel and officers from the Armed Forces of Malta’s bomb disposal unit were called in to clear the danger and certify the area safe.

Specialised equipment had to be brought in to conduct further tests. The site was declared safe by mid-afternoon when the contractor was given the all-clear to continue.

Sources said the cesspit, which did not appear on the plans, must have been there for years and was most probably excavated without a permit in the past.