Answer:
There is no actual way that the mantle could be...'exposed' or 'seen'. But scientists have managed to find multiple ways to analyze the mantle and it's actual existence by using multiple devices. The mantle is basically a 2,900 kilometers (1,802 miles) thick blocks of rocks and minerals underneath the crust(or the surface of the earth) , and makes up a whopping 84% of Earth's total volume.
Hydrogen ion, or H-, creates water when combined with a hydroxide ion, OH+.
The 'formula' for water is H20, two hydrogens and an oxygen.
I hope that I helped but if this still makes no sense, or if I haven't explained it as well, then please don't hesitate to ask further questions. Thank you.