Neptune's undersea tunnel led directly into the Deep Sea.
Compared to humans, Beastmen actually shared many of the same ways of thinking. In fact, their brains were even more developed than those of pure humans.
Therefore, they were just as curious about exploring the Deep Sea.
And so, they invented many tools for it.
In the current Interstellar era, people had already explored every corner of the seas.
This was especially true for sea regions where tunnels could be built; warriors were sent in ahead of time to clear out the Sea Beasts.
Thankfully, this was the age of Beastmen. Many of them carried the Genes of Sea Clan Alien Beasts, so fighting in the Deep Sea was not a problem.
Even the construction was thanks to these deep-sea Beastmen.
This just went to show that, in the end, the pros of genetic modification outweighed the cons.
