There is simply no reason to believe that the Earth is hallow. We've traveled 40,604 ft below the surface, so we know it is not hallow at that point. The mantle-core diagram most people are familiar with was deciphered with seismic waves and volcanic explosions. If it was hallow, gravity would be much different from what we know it to be, and looking at the gravitational pull and size of the planet, we have figured the density to about 5.515 g/cm3 (NOTE: average rocks are about 2.75 g/cm3, and if any of the Earth was hallow, that means the core would be less dense that average rocks).
I dont know if i can put this in 50 words because its just an easy answer. GRAVITY!!!! The earth would collapse and implode because of nothing to stop it from doing so. If you make a spherical object thats hollow and put an inward force upon it it will implode, if you have a spherical object that is dense you can not. (of course it all depends on materials used and amount of pressure)
Ever heard of the inner and outer core? Magma? You know, the stuff that makes tectonic plates work, where volcanoes get their lava, that stuff? The mantle has not even been scraped as ChristopherCaldwell says, 40,604 feet/7.6901515 miles. The outer crust on land is roughly 25 miles deep, so only another 17.3108485 miles to go! We have proof that the other layers are significantly LARGER. This adds up to a whopping radius of 3,959 miles (mean average)
25 miles for crust + 1802 mile mantle + roughly 1400 mile outer core + 760 mile inner core radius.
That adds up to roughly 3987 miles, note that some of these figures are rounded. Yep, that fills up the earths radius. No room for a hollow area.