Drilling into ancient ice or geological strata on Earth can be just as dangerous as bringing back a piece of Mars.

What if a virulent, long-lived bacteria or virus died out in an area for lack of hosts and was later covered with ice or sediment?

Smallpox is supposedly extinct now, except for samples in two high security labs. What if there's a nastier bug lurking beneath an arctic ice sheet? Or in a previously untapped aquifer?

In 1967, my helo squadron pulled a strike on the Mekong River Delta off the USS Iwo Jima. Suddenly our Purple 115/145 avgas turned brown. All the aircraft were affected and the ship's avgas fuel cells were also. It was caused by a virulent bacteria that was eating the fuel - tetraethyl lead and all. The ship had to go to Sasebo, Japan to get the fuel cells replaced and our birds got decontaminated at Subic Bay. In the light of the avgas eating microbe, I believe we could just as easily pull a virulent plastic-eating bug up from a new oil field as not. Then all hell would break loose!