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 post is a response to a question from a ScientificAmerican.com reader on how researchers find the places they've drilled on future trips. To track her research ship's current position, click here. To see all her posts, see "60 Seconds in North Pond."

Remember we have come back to the same patch of ocean that was originally drilled in the 1970's.  And this is not the first time scientists have been back.  Hole 395A was drilled in the mid 1970's, then CORKed in the 1990s, and then has been revisited continually to download data from the CORK head. Finding North Pond is not a problem!

That said, there have been significant advances in our ability to navigate and position a ship since then. This ship has a dynamic positioning system, which does a remarkable job of holding our position.  When we send any wire down, to do coring, or a water cast (see water sampling device to the left), or the heat flow probe, we also attach a new instrument that is being tested for geophysical measurements. It is sent down 500 meters up from the bottom, records its position, and therefore gives us some calibration of "truth" in terms of what the true position is at the bottom versus the ship position.

In general, they are very much aligned - exceptions being when we are in heavy seas, when conditions naturally cause variation in these positions.

Photo of water sampling device being launched off the Merian courtesy Katrina Edwards/USC