Enormous Ly \alpha nebulae ( ELANe ) represent the extrema of Ly \alpha nebulosities .
They have detected extents of > 200 kpc in Ly \alpha and Ly \alpha luminosities > 10 ^ { 44 } erg s ^ { -1 } .
The ELAN population is an ideal laboratory to study the interactions between galaxies and the intergalactic/circumgalactic medium ( IGM/CGM ) given their brightness and sizes .
The current sample size of ELANe is still very small , and the few z \approx 2 ELANe discovered to date are all associated with local overdensities of active galactic nuclei ( AGNs ) .
Inspired by these results , we have initiated a survey of ELANe associated with QSO pairs using the Palomar and Keck Cosmic Web Imagers ( PCWI/KCWI ) .
In this letter , we present our first result : the discovery of ELAN0101+0201 associated with a QSO pair at z = 2.45 .
Our PCWI discovery data shows that , above a 2- \sigma surface brightness of 1.2 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } , the end-to-end size of ELAN0101+0201 is \gtrsim 232 kpc .
We have conducted follow-up observations using KCWI , resolving multiple Ly \alpha emitting sources within the rectangular field-of-view of \approx 130 \times 165 projected kpc ^ { 2 } , and obtaining their emission line profiles at high signal-to-noise ratios .
Combining both KCWI and PCWI , our observations confirm that ELAN0101+0201 resides in an extremely overdense environment .
Our observations further support that a large amount of cool ( T \sim 10 ^ { 4 } K ) gas could exist in massive halos ( M \gtrsim 10 ^ { 13 } M _ { \odot } ) at z \approx 2 .
Future observations on a larger sample of similar systems will provide statistics of how cool gas is distributed in massive overdensities at high-redshift and strongly constrain the evolution of the intracluster medium ( ICM ) .