A disturbing realization has emerged from the wreckage of a deadly pipeline explosion in California: There are thousands of pipes just like it across the United States; experts say that pipes of some age were put in the ground before the dense population arrived and now the dense population is right over the pipes; thousands of pipelines across the United States fit the bill, and serious incidents are not infrequent; federal officials have recorded 2,840 significant gas pipeline accidents since 1990, more than a third causing deaths and significant injuries