Simulations predict that gas in the intergalactic medium ( IGM ) is distributed in filamentary structures that connect dense galaxy clusters and form the cosmic web .
These structures of predominantly ionized hydrogen are difficult to observe directly due to their lack of emitting regions .
We serendipitously detected an overdensity of log N ( H I ) > 18.0 absorbers at z \simeq 2.69 along the lines of sight toward a pair of background quasars .
Three main absorption regions spanning { \sim } 2 000 km s ^ { -1 } ( corresponding to 6.4 { h _ { 70 } } ^ { -1 } Mpc proper ) are coincident in the two lines of sight , which are separated by { \sim } 90 { h _ { 70 } } ^ { -1 } kpc transverse proper distance .
Two regions have [ Fe/H ] < -1.9 and correspond to mild overdensities in the IGM gas .
The third region is a sub-DLA with [ Fe/H ] = -1.1 that is probably associated with a galaxy .
We discuss the possibility that the lines of sight probe along the length of a filament or intercept a galaxy protocluster .