You may remember the New York City explosion that killed eight and injured 50 people in Harlem. An investigation into the incident, which happened about 18 months ago, revealed that the explosion was caused by a leaking sewer pipe and an improperly welded join in a gas pipe.
The sewer link caused the earth under the gas pipe to subside. The gas pipe began to bend because the earth that had previously supported it was gone. The bend was at the site of a join with bad welding that ultimately cracked, releasing natural gas into the surrounding earth and utilities.