The Buddha could not control Heaven in its entirety. If someone traveled to the area that was not under the control of the Buddha, they had a chance of making it to the other end.
Luo Ji thought the area that the Buddha did not control was under the Sand Sea.
"Once you enter the Sand Sea, you will see some white dunes. They're different from the ones of yellow sand. You won't find them if you don't look, either. Once you find them, though, head for them immediately. Then, you can enter the region that the Buddha don't control. This is merely my theory, though. There's a chance I am incorrect," Luo Ji said.
"If it's not correct, then you know you will die with me. It is best you tell me everything," Han Sen said to Luo Ji as he continued running.
"There is no more for me to tell. That is all I know," Luo Ji said.
"Really? I don't think you're the sort of person that goes on an adventure with nothing but a guess," Han Sen coldly said.