"To get back up to the shining world from there,
My guide and I went into that hidden tunnel,

And following its path, we took no care
To rest, but climbed: he first, then I-so far,

through a round aperture I saw appear,

Some of the beautiful things that Heaven bears..."

-- Dante Alighieri, Inferno

Tucked away in the Icelandic highlands, in the Reykjanes peninsula off of route 380 lies a lava tube heralded as one of the "World's most thrilling hikes" according to National Geographic, but you could never find it in a thousand years.  Luckily for the world and speleologists, Bjorn Hroarsson did his geologic homework and found something -- literally -- between 1,000 and 9,000 years old. Bjorn (the founder of my tour company) followed a hunch in the new millennium. He poked around the Leitahraun lava fields and, eventually in 2005, decided to have a closer look at a fold in the landscape. Bjorn shoved himself through an unassuming hole in the earth, obscured by boulders and only large enough for Iceland's huldufólk, and laid claim to an underground marvel.
Extreme Iceland's yellow family van on steroids bounced up the hillside making the seven of us feel like rag dolls being tossed around. Atop the bluff, the wind whipped and the sun highlighted the ocean in the distance.  Kai (from Singapore), Lorelei (from the USA), Cristobal + Nerea (from Spain), Atli + Jön (locals), and I marched through the volcanic moss along a sheep trail toward the famous Búri lava tube.
At its inngangur a.k.a. entrance I removed my backpack, put on the complimentary wool gloves, clicked the heavy LED flashlight on, cinched the chinstrap on my helmet, then lowered myself into total darkness.  With the aid of a smattering of sunlight and the intense LED lights from my fellow hikers, I saw Búri's (pronounced "Boo-ree") frozen wonderland.  Like hundreds of tree stumps rising from the earth, a montage of ice formations covered the floor.  Cold water constantly dripped from the ceiling, creating formations and columns taller than me. Though it looked crystalline and cracked, the ice was actually sturdy. We walked across thick slabs of it that coated the rocky terrain, and wide stalagmites blocked the way so our guide, Jön, lead us up a higher path which made for pretty, panoramic views.
Not far from the grand inngangur, the ice formations ceased.  We were abandoning the surface -- the only place moisture could penetrate Búri.  The toothy terrain continued deeper and remained a vibrant ochre -- a color not commonly seen above the surface in Iceland.   Due to the acute angles of the boulders, they treacherously teetered under my weight which made each step a balancing act and time-consuming process.  At some point, every one of us slipped.  Up, down, up, down.  The way stressed my knees and ankles.  There was so much labor involved in navigating the debris I was sticky with sweat & quickly shed my wool gloves.  I placed my steaming hands on the rocks used to steady myself, to absorb their coolness, but at a price: the sharp, porous lava rock blistered my thumbs & cut my palms. 
Búri certainly was not for the claustrophobic!  After the birth through the narrow passageway at the surface, space only became tighter as my group travailed onward.  Twice, we were bottle-necked to another aperture in the boulders, and the latter obstacle was downright suffocating. I imagined Bjorn arriving at this point on his initial journey & poking his head through, to affirm that Búri persisted.  Throughout the lava tube the detritus nearly touched the ceiling. I counted my helmet as the biggest blessing (over my sturdy Columbia boots, camera & water) since I repeatedly bonked my head on a multitude of natural objects: the convex ceiling, a protruding ledge, a side wall that funneled inward. Glancing ahead it was hard to differentiate between the ground, walls and ceiling, since they blended in a monochromatic sea of chaos.
Yet, the underworld revealed unexpected gems.  Other warm colors were introduced to the florid landscape.  Bright yellows & oranges mixed with the burnt sienna and resembled a fiery rainbow. The lava tunnel also yielded unusual patterns and textures: concentric circles, veins, rippled valleys, wrinkled skin, brain, streaks and sinew.  Búri was beginning to reveal its dignified wonders.
But there are plenty of places in the world to see prettily colored rock and delve beneath the Earth.  Midway, the vivid colors faded to a charcoal and the rock bore a metallic coating, almost like Quicksilver!  Unlike the jagged rocks I traversed in the past hour, the surfaces in this section of the lava tunnel were smooth with a silver glean.  One gjá a.k.a. fissure looked precisely sliced in half by the huldufólk's knife.  Here was Búri's masterpiece: sculpted by magma eras ago, silver nipples hung from the ceiling & hardened as they dripped down the walls.  Lava once violently pushed through here, covering everything in a purplish-silver gloss.  As the force and volume subsided, the lava still flowed like a river through the tunnel & the emanated heat began to melt the cooling liquid on the upper half of it.  Thus, the magma stalactites formed.  At our feet, we saw the erosive magma's path of least resistance.
With one final pitch, my group ascended a massive pile of rubble.  Abject and drenched in sweat I wasn't sure how much longer I could trod through Búri's intestines.  Like a Godsend, the floor leveled out and most of the stones that littered it thus far simply vanished.  I felt an indescribably relief to be able to hike upright & on solid ground.  If we were suffocating in the tube the last two hours, these last 200 yards felt like pure freedom!  The ceiling rose to a staggering height and the walls quadrupled in width.  This was an underground Silfra Catherdral!
We followed the same route the lava carved, photographing the brutal marks scoured into the walls.  Clearly, something large was rakishly dragged across this plane by the powerful, moving magma.  Otherwise, the tunnel was incredibly circular and could have passed for a manmade subway line. But I never saw the lights of a subway train, only the faint LED glow of my group ahead.  The camera's flash perfectly captured the reddish-brown walls, but they looked completely different illuminated by only my flashlight.
Jön came to a final halt.  A vertical, 17 meter a.k.a. 58 foot pit blocked the way. We could go no further unless -- like Bjorn -- we carried climbing gear.  However, below us & unseen, Búri endured.  After the drop off the lava tunnel burrowed for 400 meters then abruptly stopped.  Where the lava displaced, no one knew. So, we gratefully sat down, ate lunch, and shared stories by flashlight.  Positioned at the back of the group, I left a healthy distance between myself & the abyss since my LED only highlighted what it was directly aimed at.  In fact, we turned off the lights & even with eyes open, the world was the blackest of black.
After the culmination of Búri, the group & I retraced our steps, returning hours later to the picturesque Icelandic day above the surface.  Unfortunately, "The cave is now closed until further notice,"  as stated on the tour company's website; the reasoning not supplied.  Perhaps, it imploded or Bjorn returned to properly map its branches.  Regardless, Iceland's terrific geology and landscape above ground was also mirrored underground in Búri.