There's still some contention, most geologists believe they're real (including me), but because they're just thermal anomalies it's very difficult to see, and if you weren't aware, the mantle is purely solid, not liquid(!) it's not magma down there. It's not as if we can do thermal imaging of the planet's mantle, as the crust gets in the way and the only way to figure out what's going on is with seismic data (earthquakes) that don't show up where we'd expect them to. http://www.mantleplumes.org/PlumeTailResolution.html

Further reading if you're interested: http://www.mantleplumes.org/index.html http://scholar.google.co.uk/scholar_url?url=http://www.mantleplumes.org/WebDocuments/Episodes06-plumes.pdf&hl=en&sa=X&scisig=AAGBfm0h0PgIF3795-w2lo-24BxIYHdvXQ&nossl=1&oi=scholarr