No one has ever been able to drill into, let alone travel down into, the depths of the mantle itself. Mantle rock is extremely dense, owing to the increase in pressure of overlying rock material. Pressure increases with distance from the crust to the cores of the earth.
Observe the Earth's Interior image to the left where both the densities are given as well as a graph illustrating the increase in pressure with corresponding increase in depth.
Click the image to the left to enlarge it in a new window. Image Courtesy of: New York State Board of Regents Examinations 2001.