Upon reaching Erebor, the Dwarves sent Bilbo into Smaug's lair, and he was initially successful in stealing a beautiful golden cup as Smaug slept fitfully. Knowing the contents of the treasure hoard which he had slept upon for centuries to the ounce, Smaug quickly realized the cup's absence upon his awakening and searched for the thief on the Mountain. Unsuccessful, he returned to his hoard to lie in wait. Having been nearly killed in the dragon's search, the Dwarves sent Bilbo down the secret tunnel a second time. Smaug sensed Bilbo's presence immediately, even though Bilbo had rendered himself invisible with the One Ring, and accused the Hobbit (correctly) of trying to steal from him. During his discourse with the dragon, Bilbo noticed a small bare patch on Smaug's jewel-encrusted underbelly, and narrowly escaped. A thrush overheard Bilbo's account of the meeting, and learnt of the bare patch on Smaug's underside.