The Earth is deep

I've explored a few caves in the western US, and it is truly amazing to see this underside of the Earth. Even though we know that the planet is huge-- I mean, it's a sphere yet we don't sense its curve-- seeing deep holes and enormous caves still blows us away.

I'd never want to go into a volcano like this, but the pictures alone remind us that we don't live on a flat, dirt-covered plain.

http://www.dailymail.co.uk/sciencetech/article-1383001/Icelands-Thrihnukagigur-volcano-Explorers-descend-650ft-magma-chamber.html