Betsy wrote:
While pulling weeds today my foot fell into a hole. I jumped back and poked
around it finding it "hollow" underneath. I got the shovel and started
digging, and found a void about 2 feet wide and 1 foot deep. There is no
evidence of animal life in it, nor is there any debris.

Faugh. That's nothing.

1. How worried should I be? It is near the pillar supporting the post,
about 3 feet away, and about 10 feet from the foundation.

I wouldn't worry at all, if the foundation isn't cracking and the nearby
post remains plumb.

2. Should I leave it open and "watch" it or should I panic and call an
engineer or geologist or something?

Nah. Fill it in.

The house is over 100 years old. There is no apparent drain pipe in that
area, although I'm told the previous owner put some kind of exterior French
drain around the house at some point. This is Baltimore, theoretically clay
soil, although the soil in the hole is soft and sandy. It has been VERY
rainy for the last 2 years.

Well, soil does change characteristics over time. It isn't stone! The
french drain idea does sound like a possibility. We have a similar hole,
about 2' across and originally some 3-4' deep, which was located
immediately under a garden wall. It's likely some sort of washed-out dry
well, I'd guess.

We've been using it for yard waste for a while, now, especially the
junkier stuff, short of weeds, that you don't want to compost. I
recently used the chipper on a bunch of pruning offal and the chips went
in the hole, filling it up to the top for now. (Next year it'll probably
need a little more, as that composts in place and settles.)