Jake was dying to explore these underground galleries on his own, but he gave up this futile desire. He would have plenty of time to explore the hidden side of the volcano after checking out his companions' circumstances. Moreover, such an endeavour required some preparation, not to mention the fact that he was relatively dehydrated after his three-day nap.
Slightly dazzled by the bright sun shining high above him, Jake believed he had nothing to fear from these seemingly humanoid monsters for the time being. If reality was not too different from the simulation, these creatures were vulnerable to sunlight. During the day, he had virtually nothing to fear, so it made sense that they would mostly move underground, shielded from any sunlight.
