The eldest brother didn't laugh at the second brother; it all came down to who could seize the initiative first.
Looking at humanity's current state of huddling in retreat, it was already obvious who had ultimately seized that initiative back then.
The power of modern firearms could not be brought to bear against monsters, and the consequences were easy to imagine.
The monsters that emerged from the first passage had merely been ordinary monsters, yet they were already enough to become grim reapers drifting across the seas, harvesting the lives of countless travelers.
The ocean was no longer humanity's backyard. It had become a place to die.
The overlords of the previous era, the naval aircraft carriers, had also been mostly destroyed at that time. Most had been wasted without achieving any meaningful results.
The monsters emerging from just one passage had driven humanity out of the sea.
And humanity had thus lost its most precious initial window of time.
By the time humanity created mechas capable of fighting monsters, there was no longer only one tunnel in the deep sea.
There was a second, and a third.
What began with only one passage appearing in the Pacific Ocean eventually became a situation where nearly every major sea had one.
Monsters began spreading across the globe, wreaking havoc throughout the world's oceans.
Fortunately, most monsters roamed the seas, and not many came ashore to attack. That gave humanity a chance to catch its breath and adjust.
Of course, the vast treasure trove that was the ocean, occupying seventy percent of Earth's surface, was completely lost at that point.
It could be said that humanity had ceded the seventy percent of "land" that was the ocean, trading space for time to buy itself room to develop.
Yet the monster invasion continued