You'll get one that looks like a sewer with a lid and another one that is more scifi style with a glass.
Download Underground Entrances (.Sims3Packs)
These were one of the first items that I created for The Sims 3 and they are quite crappy. But as I have not seen many entrance hatches like these I though I should publish them anyway.
You can find them in outdoor decorations.
They are recolorable, but they don't have any animations. When you put them over the ladder, sims will just walk through them. They are not doors either, so visitors will happily use the ladder behind them too.