Editor's Note: University of Southern California geobiologist Katrina Edwards is taking part in a three-week drilling project at the Atlantic's North Pond—a sediment-filled valley on the ocean floor—designed to locate and study what she calls the “intraterrestrials”: the myriad microbial life-forms living inside Earth's crust. This is a response to a question from a ScientificAmerican.com reader on how long these microbes might live, and whether scientists are concerned about the effects of the test holes they're creating. To track her research ship's current position, click here. To see all her posts, see "60 Seconds in North Pond."

We typically think about "life span" in microbiology in terms of "doubling time," or how long it takes one cell to divide into two. in sediments and rocks, we don't know how long this takes. A year? 10 years? Hundreds of thousands of years? We don't understand these ecosystems well enough to say, but many of us are eager to figure it out.

A really important concept about the upper igneous rock crust - the rock that erupts at mid-ocean ridges and at volcanoes - is that it is very porous and permeable.  Water circulates through this rock readily. Minerals will eventually form from seawater and seal some of the fissures, but this takes many millions of years. Sediments that form on top of rock, in contrast, are not permeable and act like a blanket.

The flow of water into Hole 395A was tremendous, absolutely, but negligible compared to the naturally occurring inflow of water into the outcrops of rock surrounding the sediment pond. The bottom line is that the water and microbes have plenty of means to get into the crust for the past 7 million years or so, but what they're doing in there is the question.

Researchers Katrina Edwards, Jennifer Biddle and Jess Murati at the launch of the Merian, courtesy Katrina Edwards/USC