So what exactly are sewer bombs and should we be concerned?

Arbcom's master sewer bomber in disguise as Leonardo da Vinci at a Wikipedia conference.  He carries his delivery system hidden up his arse.

Sewer bombs date back to antiquity. They are thought to have been first proposed by Archimedes,  though it was left to Leonardo da Vinci to produce the first viable examples (so far this information doesn't seem to have any reliable sources).

The basic concept is straightforward. An ignition device such as a lighted candle is placed in a small receptacle such as toy boat and allowed to float through a sewer system below the enemy until it reaches a sufficiently high level of noxious gases, triggering a devastating explosion totally showering everyone for miles around with shit.

Modern delivery systems perfected by wikiterrorists such as Coat of Many Colours are thought to employ sewer drones armed with  thermonuclear triggers and are believed to possess the explosive power of many millions of evacuations.

Expert advice is above all to avoid areas such as Wikipedia conferences where there are likely to be potentially lethal concentrations of shit.