When Hopper first exits the cave, he finds his own Jeep as well as Cara's Jeep, which have been significantly overgrown with brush. This foreshadows that the time dilation is not a matter of days per second, but years per second; Hopper left the cave about thirty years after he entered.
The group finds the conquistadors locked in battle with the cavemen in the super-super-slow area near the Fountain. While it is possible that the cavemen are just hostile to all outsiders (seeing that they kill women and children), it begs a mention that from their perspective, the conquistadors, cowboys, hippies, meddling kids, and astronauts are an almost continuous flood of people entering the cave in a matter of minutes. It is understandable that they would see all as part of a single, hostile army.
The water forming on the hole above might imply that the area outside is naturally flooded now, but this wouldn't stop water from (very slowly) pouring inside. This hints that the water was actually put there and held in place with (future) technology.
The concerns all the children's families must have gone through, especially Furby's mom. The last time she saw them they were all going into a cave system and the movie makes it apparent they were never seen again.
Overlaps with Tearjerker but Hopper's dog, Boss, last saw him enter the cave. In the wilderness, miles from civilization, Boss most likely died alone and abandoned.
Boss wasn't seen nor heard by the main group which arrived nearby three days after Hopper. Not even during the whole day or two that Furby was outside the cave. Nor did Boss try to find them despite knowing Jackie and Taylor. But Furby heard a lot of coyotes around the cave...
The future humans rescued Cara, her friends, and the Hoppers presumably at Cara's request, but may have just left the cowboy, conquistadors, and cavemen down there forever. (Word of God is that the explorer was also rescued and healed in another room, as he wouldn't breathe the same air.)
