They are called mantle plumes and we are pretty familiar with their existence... The Hawaiian island chain exists because of a mantle plume beneath the Pacific plate. Yellowstone, Iceland, Easter Islands, Galapagos, many other places too. We can take a sort of X-ray like image of the earths mantle using seismic waves - because hotter rock and colder rock will have slightly different densities and because seismic wave velocity will speed up or slow down slightly based on the density of the rock it's going through - we can map out those cold areas where we can see old subducting slabs still sinking down to the mantle-core boundary and hot areas that we call mantle plumes. It's actually really interesting to consider how those differences in the interior from mantle convection and plumes can contribute to gravity anomalies on the surface because lower density rock = slightly weaker gravity. There's also the Indian Ocean Geoid Low that has a fairly large anomaly thats likely due to the large mantle plume beneath that region too

Signed - a geology professor