The Tunnel is big enough to crawl/walk with a crouched body, and is about 20 meters long. It if a fun adventure for adults and children alike, and offers a different experience inside the national park.
You can do it either as a short 1.5-2 hrs round trip, or as a loop hike. Road Trip Ryan describes on his website a scramble to the top of the mesa pass the Tunnel and descend via Forget-Me-Not Canyon. I had no time to explore that route since we (by mistake) originally searched for the Tunnel in one canyon over to the east. I would recommend a different loop - hike towards the Tunnel, climb to the mesa top and instead of hiking north, head east and descend into the canyon to the east of the one with the Tunnel formation (see the map). This loop is shorter and brings you to the same parking lot. If you decide to do the loop via Forget-Me-Not Canyon, you will end up a few miles further up the road.
Elevation at the trailhead: 4142 feet
Round Trip: 3.1 miles
Time: ~ 2 hrs
Tunnel Elevation: 4262 feet
Elevation change: 120 feet
Best Season: any time of the year, but summer can be hot, bring plenty of water.
The hike starts from the small pull-out parking lot on the west side of the road. There is a lower trail, better worn and closer to the wash - take this one. I made a mistake and took an upper trail with a post showing an image of a rappel, this took me to a side wash and to a different canyon, where I spend 3 hrs exploring and climbing to the top of the mesa. I had no route description with me or a map, just went based on my memory of reading the description on-line.
The lower trail is more worn, head along and above the north side of the wash in westerly direction. It eventually drops into the wash, and this section could be muddy (I was hiking after a rain). Aim for a cliff on the right, pass its prow (the wall is very narrow from the south), and continue inside the wash. There are several trails here. Basically, I followed some footsteps in the mud, which occasionally disappeared, but I continued west inside the wash, had to even bushwhack a little bit until the wash opened up again. The canyon is very open here. Head for the next big cliff ahead of you, you will be turning north, right before that cliff into a side canyon. I came to an area with 2 small cairns on the bottom of the wash, the trail was leaving the wash up a sandy hill to a large slick rock area. Walk north on slick rock and slightly uphill until you start to see an amphitheater on your left. There was again a cairn and trail leading towards the small mushroom rock and an opening in a rock above it. That is the entry into the Tunnel. The slick rock from the mushroom rock to the Tunnel may appear steep to some, but is not that bad. You may need to provide some partner assist to enter the Tunnel, or if alone use a couple of climbing moves. I did not find it difficult and there is a nice flat ledge just below the entry in to the Tunnel, so even if you fall from your attempt to enter the Tunnel, you will end up on this ledge and not rolling down the slick rock slope.
Once inside the tunnel crawl/walk through it all the way to the other end. You can either scramble out or return back. There is just enough light that you don't need a headlamp. The surrounding area is very scenic and fun just to sit around and relax.
If the Tunnel is your only objection, return the same way.
Loop Hike: I have not done this loop hike all the way through, about 90% of it. As above mentioned I originally explored a canyon to the east of the Tunnel canyon, and I climbed all the way to the top of the mesa from this canyon. The other canyon is basically parallel and cca 0.7 miles to the east. The East Canyon is more narrow and I would say even more pretty than the more open canyon with the Tunnel.
From the Tunnel look for the easiest path in northernly direction to the top of the Mesa. Pass through the boulder field on the way up. Once on the top, head east for cca 0.5 mile and descend into the next canyon running towards south (see photo East Canyon). Initially descend on a nice slick rock. It is easier to be on the right side lower down, but even the left side is fine if you feel comfortable on steep 4th class or very easy 5th class slick rock. The right side is more like 3rd class and easier walking terrain. You will eventually drop into a wash, which will bring you towards the main Courthouse Wash and the same parking area.
The Tunnel is located inside Arches National Park - expect to pay an entry fee. Entrance Fee is 25$ for one private vehicle, per person 10$ individuals entering either by hiking or cycling.