What percentage of the planet beneath us is inaccessible by
humans?

About 70 percent

Explanation:

Around 70 percent of the planet beneath us is inaccessible by humans. This is because a large and majority part of the planet is covered by water. This include seas, oceans, rivers etc. These are where sea animals thrive and a large amount of ocean floor has been inaccessible by humans.

However due to urbanization a larger portion of the land has been explored and been accessible to humans.

there's multiple reasons why the conquistadors were so successful in colonizing central america, and some of them are:

- they were seen as gods;

at the beginning, the spanish settlers were seen by the natives as gods, and this was mainly due to their white skin, and the way they came with their boats.

- possessed superior weaponry;

the spanish had gunpowder and guns, as well as cannons, while the natives did not had any fire weapons.

- made cleaver alliances;

they used the situation of tribes hating each other, so they made alliances with some of them which served them very well.

- brought diseases;

the native people were not immune to certain diseases the europeans brought with them, so they were dying from seeming harmless things for european standards.