97 Blazer with HUGE dark leak underneath

I have a 97 Chevy Blazer that has 180000 miles on it. We have installed a new water pump, hose, and thermostat since Christmas. I arrived at work at 8 this morning, and walked back out at 9 to get something. When I did, there was a huge, wet, dark colored puddle underneath...taking up half of the size of the vehichle. I've checked the oil...not low or dry. I've tried putting water in...it hasn't leaked out. What could this be?:runaround:

Something left by another vehicle?

Trans fluid?