This box hasn't made it onto the State page yet, but it is on the
"What's New" list:

Elephant Walk Letterbox

This park seems to have no name and perhaps belongs to the
Water Authority folks, but you can actually walk from here along
the river to Riverbend Park. No restrooms, no amenities, lots of
off-leash dogs most days. From 495, take exit 44 (Rte. 193) west
toward Great Falls. Go almost to the end, turning right on Seneca
Rd. just before the traffic light at Route 7. Drive 3.6 miles to the
end of Seneca Rd., where you will see a yellow barrier (usually
open) and a cyclone fence-type gate (closed). Park on the road.
The trail begins just south of the "No bikes on trails" sign that is
closest to the yellow barrier and gate. WARNING: In the Fall,
these trails can be VERY hard to see because of all the fallen
leaves. Take your time! Go 40 paces to a fork just past a fallen
tree on the left and follow the path that bears nearly due North.
Stay on this path past where another trail comes in on the right,
with a "No bikes" sign, and a service road comes in on the left.
Stay on this now-broadening trail while it turns nearly due East
and heads downhill. After 10 minutes or so, you will hear the
river ahead. When the trail flattens out and turns right to follow
the river, go about 45 paces to a path on the left that leads to the
river and a nice picnic spot. Enjoy the spot and your lunch.
Return to the broad path and keep going downstream as the trail
gradually moves away from the river. After about 25-30 minutes
total (from your car) you'll come to a place where 2 trails emerge
on the right. Take the one that bears first about 260 degrees and
then quickly turns to 270. Follow this path up past the logs
across the trail, past a large grapevine on the left and then a
double-trunked tree on the left with, on the right, a tree with very
pitted bark on the trail side (woodpecker holes?). Stand on the
trail between these 2 trees. On a bearing of 294 degrees, walk to
a tree with a bulge at the base. Elephant Walk Letterbox is at the
base of this tree under a rock, leaves, sticks and a small rotted
log.
Difficulty: Aboaut 30 minutes one way, with a short, STEEP uphill
at one end, and a longish uphill on the return. You will need a
compass, and you will need to differentiate between a step and
a pace.