Start of the Big Hollow Trail in the backcountry of Mammoth Cave National Park

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If you plan to hike in the backcountry of Mammoth Cave National Park, do not rely on the map that comes with the free park brochure. Purchase the weatherproof map by National Geographic that is available in the Visitor Center book store (it is also available from various online retailers). This map has more detail, and it is going to last much longer than the paper map. I hiked most trails in the backcountry using the National Geographic map, and it’s nearly as good as new.

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• Length:  10.4-mile loop plus 2 miles round trip on the Maple Springs Trail
• Time:  4-5 hours on foot, 3 hours on bike (with Maple Springs Trail)
• Difficulty: North Loop is easy to moderate for both hikers and bikers; South Loop is moderate for hikers and moderate to strenuous for bikers.

The Big Hollow Trail in the backcountry of Mammoth Cave National Park is a 10.4-mile loop trail that is accessed from the Maple Springs Trailhead. You must first travel one mile on the Maple Springs Trail to reach it. Both trails are open to hikers and bikers. The Maple Springs Trail also allows horseback riders.

Big Hollow Trail map (click to enlarge)

The Maple Springs Trailhead is located on the north side of the Green River just up the road from the Green River Ferry. If the ferry is operating, you can get to it in 15 minutes or so from the Mammoth Cave Visitor Center or the Mammoth Cave Campground. If it is not running due to low water—which happens all the time—it is an hour drive from the Visitor Center.

The Big Hollow Trail is unique in that it is the only true mountain biking trail in Mammoth Cave National Park. It is comprised of two loops. If you hike or bike only the North Loop, the distance is 5.3 miles. The South Loop adds 4.8 miles. There is no way to get to the South Loop without first taking part of the North Loop.

I rode my bike on the trail, so this review is geared more for bikers than hikers, but the description and photos will give everyone a good idea of what to expect. Overall, it is a fun biking trail for most everyone, but it’s probably too difficult for young children.

As mentioned, a hike or bike ride on the Big Hollow Trail actually starts out on the Maple Springs Trail. When at the Maple Springs Trailhead parking lot, look for the restroom. The Maple Springs Trail starts to the left of the building and runs for a mile before coming to the intersection with the Mill Branch Trail (left) and a .3-mile connector to the North Loop of the Big Hollow Trail (right).

Intersection of the Maple Springs, Mill Branch, and Big Hollow trails in the backcountry of Mammoth Cave National Park

The connector has a relatively smooth surface. It heads uphill for the first .2 mile, but the incline is so minimal that it might as well be flat. The last tenth of a mile is downhill, and the slope is a little more substantial. I was able to coast all the way to the bottom.

Typical terrain on the first segment of the Big Hollow Trail in the backcountry of Mammoth Cave National Park

At the start of the North Loop, I took a left and headed around in the clockwise direction (east side of the loop), and this report is written from that perspective. If all you want to do is get to the South Loop for some reason, it is quicker to stay to the right, otherwise it doesn’t make any difference which way you go.

Start of the Big Hollow Trail’s North Loop, Mammoth Cave National Park

The general trajectory of the first mile of the North Loop’s east side is downhill, but again, most of the hills are not very big. The steepest rarely have grades of more than -10% (minus denotes a downhill grade). I coasted most of the way down, and I got up the small uphill segments pretty much using my downhill momentum.

The east side of the North Loop is narrow and has its share of roots and rocks. However, doing the bike ride in the fall presents another challenge—acorns. It was often like riding on marbles, and my bike nearly slipped out from under me on two occasions when going around curves in the trail.

Typical terrain of the first mile of the east side of the Big Hollow Trail’s North Loop, Mammoth Cave National Park

The reason for the largely downhill ride is because there is a creek at the bottom, and waterways tend to be at the lowest point on the landscape. There is a short-but-steep downhill run just prior to the creek, which was dry when I did the hike.

Footbridge over a small creek on the east side of the Big Hollow Trail’s North Loop, Mammoth Cave National Park

A mile from the creek is the intersection with a shortcut trail that lops 1.1 mile off the North Loop. According to the sign at the intersection, a left leads to the SOUTH LOOP and a right leads to the SOUTH LOOP VIA SHORTCUT. If you want to stick to the official North Loop, which is the long way, take a left. Referencing the trail map, this is the section towards the bottom of the North Loop that sticks out like its own little side loop. If you want to skip this side loop and shave off 1.1 mile, go to the right on the shortcut trail.

Intersection on the east side of the Big Hollow Trail’s North Loop with a shortcut trail to the South Loop, Mammoth Cave National Park

I ultimately took the long way around (left), but I also checked out the shortcut trail since it isn’t very long. It has an overall uphill trajectory, though there are only two short hills of any note. The rocks are a little more prevalent. They aren’t big, but they do protrude from the ground just enough to jostle you around when biking. Hikers won’t notice a thing.

The side loop is more of the same. Other than a 100-yard downhill stretch with an average grade of -7% (dry creek at the bottom), the trail is a series of ups and downs, some minor like a kiddie rollercoaster and some lasting as long as 100 or more yards. The three largest ascents / descents—up and over one hill and up the next—come right before the intersection with the connector to the South Loop. The last hill has an 11% grade, so it is quite difficult on a bike (10% is the start of strenuous biking whereas 15% is the start of strenuous hiking). The trail runs along a ridge for most of the way, so any sort of crash could send you tumbling down to the bottom of a ravine.

Last mile on the east side of the Big Hollow Trail’s North Loop follows along a ridge, Mammoth Cave National Park

A 1-mile connector trail runs between the North and South loops of the Big Hollow Trail. Heading around the east side of the North Loop as I did, the connector comes 3.5 miles from the start of the loop (not the Maple Springs Trail). If you take a right to continue around the North Loop, skipping the South Loop, you will be back at the parking lot in about 3 miles. Stay straight to get to the South Loop.

North end of the connector between the North and South loops of the Big Hollow Trail in the backcountry of Mammoth Cave National Park

The connector trail has one steep-but-short downhill segment—of course, uphill on the way back—but other than that it’s pretty easy.

Typical terrain on the connector between the North and South loops of the Big Hollow Trail in the backcountry of Mammoth Cave National Park

When I reached the fork to begin the South Loop, I stuck with my clockwise travel by taking a left. Clockwise had been good to me so far. It may be my imagination, but there seemed to be a lot more downhill segments than uphill, at least to this point.

Start of the Big Hollow Trail’s South Loop in the backcountry of Mammoth Cave National Park

The terrain doesn’t change much on the east side of the South Loop. It starts off with three consecutive kiddie-rollercoaster hills all the while continuing to run along a ridge, so a crash could still spell disaster.

Typical terrain at the start of the Big Hollow Trail’s South Loop when heading around in the clockwise direction, Mammoth Cave National Park

After the three hills comes what is so far the longest downhill segment on the trail at a quarter mile. There are a few short segments with grades greater than -10%, so I wouldn’t want to be coming the other way around and having to bike up these hills. As I said, clockwise had done me right, and it continued to do so.

Once at the bottom of the hill (half mile from the start of the South Loop), the Big Hollow Trail levels out a good bit, but it does get very rocky for the next half mile and is substantially rougher than anything that came before. There are rocks you would have to step up onto if hiking, and some are located on uphill segments of the trail. A few times they brought me to a stop, and I had to get off my bike and push it over this terrain. I can’t imagine the typical biker being able to ride up and over these rocks any more than I could.

Rocky section on the east side of the Big Hollow Trail’s South Loop, Mammoth Cave National Park

At the very southern end of the South Loop, right when the trail curves and begins heading back north, things change for the worse. I should have guessed, because if the east side of the trail is largely downhill, there has to be an uphill section somewhere. There are two small hills with grades of 10% and a quarter mile uphill climb with an average grade of 11%. There is even a 100-foot section with a 20% grade. That’s tippy-toe steep even when hiking, so all but expert bikers in great shape will be pushing their bikes up this section. The hilly terrain lasts nearly all the way back to the start of the loop. Only the last .3 mile is downhill. Needless to say, the west side of the South Loop is by far the most difficult section of the Big Hollow Trail when traveling in the clockwise direction.

Steep terrain at the southwestern end of the Big Hollow Trail’s South Loop in Mammoth Cave National Park

The intersection with the connector trail back to the North Loop comes 7.3 miles from the start of the Big Hollow Trail. Stay left at the fork, otherwise you’ll be on your way around the South Loop again.

Southern end of the connector between the North and South loops of the Big Hollow Trail in the backcountry of Mammoth Cave National Park

After a mile on the connector, which is familiar territory, comes the intersection with the North Loop at its southern end. The directional signage at the intersection is very confusing. I couldn’t make heads or tails of it. There are two signs, with the first stating that this is the BIG HOLLOW TRAIL NORTH LOOP. Clear enough. The second sign, which is the one that makes no sense, indicates that the NORTH LOOP SHORTCUT goes both to the left and to the right, which is impossible as far as I can tell. Just ignore this sign. A right is the North Loop in the counterclockwise direction along what I earlier called the side loop. A left, which is the correct way to go, is the North Loop in the clockwise direction (west side). The intersection with the shortcut trail is actually a tenth of a mile farther ahead. When you get there, stay left to continue on the west side of the North Loop.

Intersection with the North Loop at the northern end of the connector trail between the North and South loops of the Big Hollow Trail, Mammoth Cave National Park

The first .8 mile of the North Loop is once again filled with little ups and downs but overall easy to hike or bike. The surface is a little rocky, but there are no rocks like on the South Loop, just rocks that jostle you around on a bike. Hikers won’t notice much of anything.

Typical terrain at the southwestern end of the Big Hollow Trail’s North Loop in the backcountry of Mammoth Cave National Park

The last three quarters of a mile before reaching the top end of the North Loop is once again hilly, with the general trajectory being uphill. Overall there are four hills with slight downhill or level sections in between, but none of them are anything like the hills on the South Loop. The first hill starts off with a 125-foot / 10% grade climb, but the remaining 500 feet is much easier. The second hill has a very short segment with a 10% grade, but overall the grade is 6%. The grade of the third hill, which is less than 100 yards to the top, is also 6%. The last hill, which is really a bunch of short hills that come one right after another, also has a short segment with a 10% grade, but the rest is easy. When you get back to the start of the loop, just continue straight ahead to connect to the Maple Springs Trail.

Hilly terrain at the northwestern end of the Big Hollow Trail’s North Loop in the backcountry of Mammoth Cave National Park

Overall, this is a great mountain biking trail for teens and up, but I don’t suggest this for a family outing with small children because the tots will be pushing their bikes much of the way. Even doing just the North Loop is too much. I’m old, and it was a little tough for me, but still enjoyable.

As for those on foot, this is an excellent hiking trail. No part of it is overgrown, and since horses don’t travel on it, there are no mudholes that have been created by horse hooves churning up the dirt in wet areas, something that plagues many of the backcountry trails in Mammoth Cave National Park.

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