The tiny planet Mercury began crossing in front of the sun (from Earth's perspective) at 7 a.m. Eastern Monday.
This sort of thing only happens a few times in a century.
You still have a chance to watch it – it takes 7 hours. But you need to be careful to avoid damaging your eyes. The simplest way to view it is by using a pair of binoculars as a projector. DO NOT LOOK AT THE SUN THROUGH BINOCULARS – simply point the binoculars toward the sun an allow the sunlight to project through them onto a piece of paper or a wall. You'll see an image of the sun projected and you can safely see the transit.
Or, check out this timelapse video from NASA's Solar Dynamics Observatory, which shows the transit so far: