If you think threading a needle is difficult, imagine drilling into an 18-centimetre-wide cylinder 5500 metres below the sea floor.
That is the challenge facing BP engineers as they attempt to drill two relief wells in the Gulf of Mexico in a bid to stem the flow of oil from the stricken Deepwater Horizon. A relief well aims to bisect the original well casing, enabling engineers to pump in mud and concrete to seal up the well.
All previous attempts to plug the leaking well have failed, so relief wells are the last resort, says Greg McCormack at the University of Texas at Austin. 'There isn't anything else you can do.'
To reach the damaged well, the engineers are using a system called Measurement While Drilling (MWD) - accelerometers and magnetometers attached to the drill bit monitor the movement and direction of the wellbore and transmit this information back to the surface.
The system sends these messages by creating pulses in the flow of mud that is constantly pumped in and out of the borehole to clear away the rock cuttings, says Ken Arnold, who runs KACI, an oil industry consultancy in Houston, Texas.