Arne Saknussemm, who wrote it during the fifteen hundreds.  Otto’s assistant Axel decodes the journal and learns of a secret passageway to the center of the Earth, deep into the Earth’s core. 

Otto hires a guide, Hans. They follow Saknussemm’s directions, and begin at a crater on Mount Sneffels.  The journey is more treacherous than they expected.

"…There are about three hundred volcanoes on the whole surface of the globe—but the greater number are extinct. Of these Sneffels is one. No eruption has occurred since 1219—in fact it has ceased to be a volcano at all." (Ch. 4)

Although Saknussemm is a heretic and an alchemist, he seems to have known what he was talking about.  The team is able to follow the directions and tunnel into the earth.

 As far as we had hitherto gone, facts had proved the theories of Davy and of Lidenbrock to be correct. Until now, all the peculiar conditions of refractory rocks, of electricity, of magnetism, had modified the general laws of nature, and had created for us a moderate temperature… (Ch. 41)

Inside of the Earth they face treacherous winds, and when they finally get outside they barely escape with their lives.  Although they only went inside a volcano, their feat is still celebrated and revered.  Axel decides to write about the journey, which is the manuscript you read as Journey to the Centre of the Earth.

The major complication might be that the team never really made it to the center of the Earth, so to speak.  However, they did make it deep into the Earth, into the crater or the volcano.  They faced danger and treachery at every turn, and came out alive and celebrated.

If you are hoping for action packed sequences like what happen in an Avengers movie, then you are not going to find them in this book.  That doesn't mean there are not exciting and tense parts though.  The action is focused squarely on man vs nature in this Verne book. Additionally, some of the most tense parts for me are more cerebral than action packed.   

For example, when the group begins their journey toward the center of the earth, they are descending down through the throat of a volcano.  The volcano is supposedly extinct, and that should create less anxiety, but be honest, would you want to descend through earth's crust through a volcano?

The underground storm sequence is fairly action packed too I think.  What's great about that part is how Verne builds up to it.  Verne lays down so much anticipation to the impending storm that the reader can't help but feel excited and apprehensive about what's coming.  

‘We’re going to have a storm,’ I continue, pointing towards the horizon. ‘These clouds are lowering upon the sea, as if to crush it.’

Once the storm hits, it's not some quick rain storm that's over in a few hours.  It lasts for days.   The narrator's description of the storm is vivid, exciting, and action packed.  

The mass of water vapour becomes incandescent; the hailstones striking the metal of our tools and firearms become luminous; each of the waves surging up resembles a fire-breathing breast, in which seethes an internal radiance, with each peak surmounted by plumes of flames.

There's lots of exciting action stuff after the storm too.  Explosives, earthquakes, and surviving the volcanic eruption that spits them out.