Nether Fortresses and Strongholds gernerated in 13w01b have some invisible parts.
And the normal terain dont gets deletet properly,
so in the Fortress or Stronghold some part are just netherrack or Stone(dont drop Blocks but stay if you reload)
if you update the Blocks they turn into what they normaly would be(Netherbrik, Stonebrick of Endpotal Frame) Lava dont seem to be efectet.
This Block do exsits really you can walk on them.
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Duplicate of MC-5737.