Description of Nainital is there in Manas Khand of Skand Purana.
It was referred to as "Tre Rishi Sarovar",or the lake of three sages,namely ATRI,PULASTYA and PULAHA.It is believed that these sages came here on a piligrimage.They felt thirsty and as they found that there was no water,they dug a hole and siphoned water,from the holy mansarovar lake.