TORONTO - The arrest of two people emerging from a manhole in downtown Toronto on Sunday cast the spotlight on a mundane but critical piece of the G20 security puzzle.

In Toronto, as in cities across North America, manhole covers provide access to vital infrastructure lifelines, such as telecommunications and utility lines.

They can also be the ideal place to hide an explosive device.

"It is quite a pre-occupation because we don't know what people will be doing," said Ottawa security expert Michel Juneau-Katsuya.

"In the psychology/dynamic of the confrontation that is currently taking place, absolutely anything goes -- damage and vandalism is the name of the game."

Early Sunday, two security guards saw two men emerging from the sewers on Queen Street -- scene of much of Saturday's violence -- and alerted police. Four people were arrested in the incident.

Summit security spokesmen said they did not know what the pair were doing underground but confirmed their belief that the incident was G20 related.

However, they insisted there was no larger security threat.

"At no time was there a risk to the safety of summit participants," said Neena Snyder, a spokeswoman for the Integrated Security Unit.

She had no further information on those arrested.

The vulnerabilities have long been recognized, with the Dept. of Homeland Security in the United States issuing guidance on protecting manholes -- especially when close to critical infrastructure facilities.

One common way of dealing with the issue is simply to weld the heavy metal covers shut. This is often done in high-risk areas or on routes used for convoys carrying high-ranking officials.

Toronto city workers spent much of Sunday morning welding shut manhole covers in the protest zone "as a precaution" amid police concerns over more protests and violence.

Although quick and effective for securing the immediate threat, welding makes it much more difficult to gain access to the manhole in an emergency.

As a result, some companies, seeing a hole in the market, have developed specialized locking devices that prevent unauthorized access but don't get in the way of legitimate use.

Various commercial methods are now available, ranging from simple motion sensors to sophisticated locking systems.

Juneau-Katsuya said the heavy security presence above ground might have prompted some of the more extreme protesters to try take their fight below the surface.

However, for demonstrators to pull off a real attack underground -- especially one that would have the required visibility -- would be difficult, he said.

"It would necessitate people being capable to either pop out into the security zone ... or have substantial amount of explosives," he said.

"This threat is more to cause trouble, to cause damage so they get noticed," he said.