Mount Etna lava flow
Mount Etna, Sicily
Often lava doesn't flow on the surface like in the pictures before, but it flows in so called lava tubes. Lava on the surface of the flow cools down and builds a crust, while in the inner part, the lava stays hot and flows on quickly.
Sometime the crust breaks though. This is what is called a skylight then. You can approach it and take a peek to see the stream of red-hot glowing lava rapidly flowing inside.
A distance of about 2-3m (6-9 ft.) is how close you can go. And even then you can hardly stand watching longer than about 15 to 20 seconds, because your eyes and face will start to burn.
The lava inside will be at a temperature of about 1200°C (2200°F)