Lava falling from fountains and issuing quietly from vents often forms incandescent lava streams or lava flows, leading to the colorful term "rivers of fire," often used in popular accounts of Hawaiian eruptions. During some Mauna Loa eruptions, several lava flows rushed down the steep slopes at 35 miles per hour! During long-lived eruptions, lava flows tend to become "channeled" into a few main streams. Overflows of lava from these streams solidify quickly and plaster on to the channel walls, building natural levees or ramparts that allow the level of the lava to be raised. Lava streams that flow steadily in a confined channel for many hours to days may develop a solid crust or roof and thus change gradually into streams within lava tubes. Because the walls and roofs of such tubes are good thermal insulators, lava flowing through them can remain hot and fluid much longer than surface flows. Tube-fed lava can be transported for great distances from the eruption sites. For example, during the 1969-74 Mauna Ulu eruptions at Kilauea, lava flows traveled underground through a lava-tube system more than 7 miles long to enter the ocean on five occasions.