Ohh, wait, you meant flat smooth, rather than curved smooth. My bad. Didn't quite pick that up the first time. I guess that's what I get for reading out of order. Oops.

I meant internally smooth spherical-ish craters created by EDM in the lab. As opposed to smooth/flat as in a calm lake or a sheet of ice...

Anywho, as regards the fluid versus other hypotheses debate...

A couple images from Nasa of Titan:

(notice the "channels" running "through"/"over" the dark circular patch mid-image. And in the lower right circular depression at which a "channel" system seems to terminate. Though the circular depression seems more like a moat than a lake due to shading. And I don't know of many fluids that naturally flow in a circle...? Also note the copious black streaks in the upper/lower left akin to dust devil tracks which we know are created electrically by interaction on Mars, and have little reason do doubt that a dissimilar process would create similar streaking here...)
http://photojournal.jpl.nasa.gov/catalog/PIA03555

(Nitice the crater here. Theory goes that what, methane/ethane drizzle out of the atmosphere and pool or channel in depressions, right? But does this crater look like it's got fluids in it? Not especially... Could be a trick of the instrument used, being a radar mapper. But on the other hand, could just as well be that there's no fluids?)
http://photojournal.jpl.nasa.gov/catalog/PIA07368

(Again, hard to tell if there's any fluids here or not, since it's a radar image. Though the lighter spots seem to be running through the darker spots. And aren't the darker spots usually interpreted as lakes, rivers or whatnot?)
http://photojournal.jpl.nasa.gov/catalog/PIA07366

(Again, hard to really tell from radar images what's in these. But rather than havign actual measurements of methane/ethane or any flowing fluids, Nasa has still hypothesized large flowing rivers and erosive tributary systems...)
http://photojournal.jpl.nasa.gov/catalog/PIA03564

(Again, they seem to like the river/tributary system model.)
http://photojournal.jpl.nasa.gov/catalog/PIA07238

Counter-point.

Compare the above river/tributary system with the below:
http://www.celestialmonochord.org/2006/05/sidewalk_fulgar.html

When it comes right down to it, these look A LOT alike. On both, we've got a central nearly linear "channel" and "secondary channels" at right angles, which is a well known configuration of electrical scarring.

So, tell ya' what, if I were to radar map the lightning strike in the sidewalk, could you explain to me how water or methan/ethane eroded it? I bet someone would. Anyone who did would be dead wrong...

There's not always only one possible explanation for features. Some will argue that erosion by fluids is more convincing or easier. Fine, but other options do exist, if we go looking. And do equally fit the final product (large ravines, with secondaries running off ar right angles, and branching further from there).

Abnyway, my point is that, not all places with craters or depressions seem to have the expected look of lakes filled with fluids, with or without inflow/outflow channels. Though, if it's drizzling methane/ethane fairly frequently or all the time, wouldn't we expect for it to eventually build up in most if not all depressions of any size? If it doesn't, does that tell us anything useful? Or would we then equivocate and say, well, maybe it rains preferentially only in these systems that we think look like lakes/rivers?

I know, I know, i'm the lone voice of dissent, and whatnot. I'll be quit now, for a little while anyway. Well, I'll try anyway. Though dunno if it'll work, since people seems to like to think inside their little boxes and never outide of them. *wink* Sometimes I feel it necessary to challenge people's assumptions, and poke holes in the boxes to make people see what's outside and perhaps think slightly. Even if they still refuse to come out and play.

*Off his soap box, and off his rocker...* =o]