Spawn in the underground is very rare in MCPE. Here is a seed, you spawn in the a cave in the underground with some lava, in this seed you will find lots of diamonds, gold and other rare ores… (but sometimes you will meet a issue made you spawn in front of an waterfall, if that happen, create a new world). Before using this seed in survival mode, you need playing it in creative mode first. Because sometime the lava pool close-up your path and you need creative the walking path with some cobblestone to passing it.
For more information please watch this video:
Seeds : glowifics diamonds Work in PE ver0.10.5 or ver0.11.0+