Frontier

 The frontier is still here it is space, underground, and in the ocean. Space is not the only frontier left for instance there is only 1% of the ocean that has been explored. We have hardly just cracked the surface of underground discovery, sure we have been in tombs and mines but that isn't all of underground there is still frontier there left to explore. We still even have land left to explore we find new species of animals almost every day and in different places. Space is almost endless us humans don't have the time to live and tell the tale to get to one galaxy to the other. Space is almost as endless as the oceans and underground is almost as dense as the forest we explore every day, so the answer is yes there is still frontier left to explore.